referring now to the drawings , fig1 and 2a show a medical wrap 2 , specifically an elbow joint wrap , comprising an outer envelope 4 , formed from a liquid impervious material which may be a polymeric material such as a butadiene - acrylonitrile copolymer , a polyester such as polyethylene terephthalate or vinyl polymer such as plasticized polyvinyl chloride , plasticized polyvinylidene chloride , low and high density polyethylene and ethylene - vinylacetate copolymers or a metal foil such as aluminum foil . housed within the liquid impervious outer envelope is a powder - like mix comprising a silica matrix 8 containing a phase change material 6 . the pcm / silica dry powder is shown diagrammatically only . in actual practice the phase change material is absorbed or adsorbed within and throughout the porous structure of the silica matrix . in any event the medical wrap 2 may also comprise fastener means such as &# 34 ; velcro &# 34 ; strips ( not shown ) to provide for attachment of the wrap around the desired anatomical body part . the preferred silica for use in the pcm / silica dry powder is a hydrophobic silica that has been surface treated with 0 . 5 - 5 pph ( parts per hundred by weight ) of a reacted silicone resin or a silane coupling agent such as dimethyldichlorosilane . as used herein in the specification and claims , hydrophobic silica is used to refer to a silica wherein the surface hydroxyl groups normally present have been reacted with silicone resins or silane coupling agents to form a less polar &# 34 ; hydrophobic &# 34 ; surface . the silica particles treated in such a manner may be either fumed silicas or precipitated silicas . exemplary silicas include precipitated silicas such as those disclosed in u . s . pat . nos . 5 , 106 , 520 and 5 , 282 , 994 , incorporated herein by reference . the preferred silica particle size is from about 7 × 10 - 3 to about 7 × 10 - 2 microns . in fig3 a - 3d the liquid activating solution , such as water , for activating the endothermic or exothermic activity of the composition for thermal energy generation fills a small , sealed , liquid - tight inner bag 10 of suitable flexible plastic that is relatively easily ruptured by the liquid when squeezed manually . inner bag 10 , filed with the liquid activating solution is inserted into an open end of an open - topped intermediate envelope 11 which may be made of a liquid impervious material as in fig1 and 2a . the remainder of envelope 11 is then filled or substantially filled with composition 12 of the present invention , in this instance a silica based dry powder containing an endothermic or exothermic compound for thermal energy generation . following that , the top of intermediate envelope 11 is sealed . when the squeezing occurs , inner bag 10 is ruptured , enabling the liquid activating solution to mix with the silica based dry powder containing the endothermic or exothermic compound , causing the latter to absorb or release heat , depending on the type of material it is , but the activated mixture remains sealed in intermediate envelope 11 . after being filled , as described , and before being squeezed to absorb or release heat , the sealed intermediate envelope 11 is slidably inserted into an open - topped outer pouch 13 , that is just slightly larger in size . preferably , the outer pouch 13 is of flexible , transparent plastic , such as polyethylene or other impervious material as in fig1 and 2a , and it is relatively flat , with its opposite major faces joined integrally to one another at the opposite side edges 14 and 15 and sealed to each other along the bottom of edge 16 . a thin , flat , flexible sheet 17 of heat insulation material may be inserted into the outer pouch 13 either before or after the insertion of the sealed intermediate envelope 11 . this heat insulation sheet may be of fine cell or cross - linked polyethylene or other suitable material , and it extends substantially completely across the inside of one major face of the outer pouch 13 and separates this side of the pouch from the sealed intermediate envelope 11 . after both the insulation sheet 17 and the sealed inner envelope 11 have been inserted , the top of the outer pouch 13 is sealed to provide a liquid - tight package . preferably , the outer pouch 13 is of suitable flexible , transparent plastic and , in the absence of a defect , it does not rupture when squeezed manually . the outer pouch 13 constitutes a means for affixing the insulation sheet 17 to envelope 11 . the affixing means could take other forms such as adhesive , but the outer pouch is preferred and advantageous . for example , the outer pouch provides extra protection against leakage . before using this instant cold or instant hot pack to either heat or cool , the pack provides a relatively flat package containing the sealed inner envelope 11 in which the particles of composition 12 are segregated from the liquid activating solution by the rupturable membrane constituted by the inner bag 10 . after the thermal energy generation upon first use , the mixture in intermediate envelope 11 may serve as a thermal energy storage device , and by heating or cooling the pack , as the case may be , it may be reused for medical therapy or other uses . thus , the liquid activating agent may itself be a phase change material which not only activates the endothermic or exothermic compound , but is also absorbed or adsorbed by the silica particles so as to form a pcm / silica gel or dry powder capable of thermal energy storage upon reuse . fig4 and 5 depict a tableware item , a dinner serving tray 30 of the type used by airlines , etc . that incorporates the present composition disposed therein . tray 30 comprises a plurality of compartments 36a - d to act as receptacles for food and a beverage container . preferably , tray 30 comprises a plastic housing 34 that is filled with composition 38 of the present invention which in one embodiment is a water / urea pcm / silica dry powder composition and in another embodiment is a silica based dry powder containing an endothermic or exothermic compound . in that instance , a tap 40 may be used for addition of a liquid activating solution such as water in order to initiate the endothermic or exothermic reaction . alternatively , a liquid activating solution such as water may be contained in cavity 32 separated from composition 38 by a rupturable divider ( not shown ), such as a rupturable membrane , which is ruptured to initiate the endothermic or exothermic reaction . as with the embodiment employing a water / urea phase change material , other uses for an instant cold system , could be for ice cream freezers and other refrigeration devices . with the instant hot system , the device would be to heat food or drink . other uses for an instant hot system of this type would be stadium seats , boots , vests , caps , ear muffs , scarves , etc . in order that the invention may be more readily understood , reference is made to the following examples , which are intended to be illustrative of the invention , but are not intended to be limiting in scope . this example illustrates the general laboratory procedure for preparing a water / urea clathrate or inclusion compound . commercial chemical and medical grades of urea were mixed with water in ratios ranging from 90 / 10 % water / urea by weight to 54 . 5 / 45 . 5 % by weight . all of the samples except for the 54 . 5 / 45 . 5 % ratio dissolved quickly at room temperature . this sample required heating to obtain a solution that did not appear to be complete when the sample cooled down to room temperature . analysis of thermal energy storage characteristics of the solutions was performed . the compositions containing water / urea in ratios of 30 / 1 down to 13 / 1 showed two distinct melting and freezing temperatures , one of which was essentially water , and the second apparently a clathrate or inclusion compound with a melting temperature of about - 11 ° c . and a freezing temperature of about - 15 ° c . as the water content in the composition was decreased to water / urea 13 / 1 and lower , the melting and freezing attributed to a separate water phase disappeared altogether , and a single clathrate melting and freezing near - 11 ° c . and - 15 ° c . respectively remained . further , the thermal energy storage characteristics of the water / urea phase change materials were about 70 cal / gm , which is close to the accepted value of 80 cal / gm attributed to water . comparisons were made with water modified with ethanol at compositions of water / ethanol of 48 / 1 to 12 / 1 . such compositions exhibited normal freezing point depression and a progressively lower heat of fusion as more ethanol was added to bring the molar composition to water / ethanol 12 / 1 . the data regarding thermal energy storage characteristics is summarized in tables 1 and 2 . wherein the differential scanning calorimetry ( dsc ) data includes the melting temperature in degrees centigrade ( tm ° c . ), the freezing temperature in degrees centigrade ( tc ° c . ), the difference between melting and freezing temperature ( tm - tc ° c . ), the heat of fusion in calories per gram ( δhf cal / gm ) and the heat of crystallization in calories per gram ( δhc cal / gm ). table 1______________________________________ δhcmaterial tm ° c . tc ° c . tm - tc ° c . δhf cal / gm cal / gm______________________________________h . sub . 2 o / etoh - 2 . 8 - 14 . 2 16 . 2 38 . 8 40 . 195 / 55 wt . 48 . 8 / 1 molarh . sub . 2 o / etoh - 5 . 7 - 15 . 3 9 . 6 30 . 6 31 . 590 / 10 wt . 23 / 1 molarh . sub . 2 o / etoh - 7 . 4 - 16 . 1 8 . 7 27 . 3 27 . 885 / 15 wt . 14 . 5 / 1 molarh . sub . 2 o / etoh - 10 . 8 - 21 . 6 10 . 8 22 . 1 22 . 082 . 5 / 17 . 512 / 1 molar______________________________________ table 2______________________________________ δhcmaterial tm ° c . tc ° c . tm - tc ° c . δhf cal / gm cal / gm______________________________________water / urea - 11 . 04 - 20 . 80 9 . 76 68 . 73 62 . 8690 / 10 wt . - 3 . 37 - 14 . 00 10 . 7230 / 1 molarwater / urea - 11 . 33 - 23 . 49 12 . 15 58 . 39 51 . 9285 / 15 wt . - 6 . 99 - 11 . 51 9 . 6417 . 7 / 1 molarwater / urea - 11 . 01 - 16 . 54 5 . 62 62 . 79 59 . 7680 / 20 wt . - 14 . 73 3 . 3213 . 3 / 1 molar______________________________________ several samples of water / urea phase change material / hydrophilic silica ( cabot ms - 7 ), i . e . pcm / silica dry powder , were prepared using the general mixing procedure described in related u . s . pat . no . 5 , 282 , 994 . the composition of the clathrate portion was varied to include water / urea ratios of 13 . 3 / 1 , 13 . 0 / 1 , 12 . 5 / 1 and 12 . 0 / 1 . free - flowing dry powders were formed in each case at pcm / silica compositions of 60 / 40 % by weight . thermal energy storage was analyzed by differential scanning calorimetry , and the data is shown in table 4 in the same manner as the data , shown in tables 1 and 2 . it was observed that all of the samples showed an undesirable low freezing temperature of about - 25 ° c ., versus about - 15 ° c . for the 100 % clathrate of the same composition . additionally , the thermal energy storage was at a significantly lower value of 30 cal / gram , whereas the storage predicted from the value of the neat water / urea phase change material , multiplied by the percentage of phase change material in the pcm / silica dry powder should have been about 40 cal / gram . it was concluded that the hydrophilic fumed silica was bonding with some of the water and thereby destroying the stoichiometry of the water / urea phase change material . table 4______________________________________ δhcmaterial tm ° c . tc ° c . tm - tc ° c . δhf cal / gm cal / gm______________________________________water / urea - 13 . 7 - 25 . 4 11 . 7 29 . 2 28 . 8pcm80 / 20 wt . pcm / silica60 / 40______________________________________ using the general mixing procedure described in u . s . pat . no . 5 , 282 , 994 , a sample of water / urea phase change material ( 10 / 1 molar ) was mixed with ppg bxs 318 from ppg industries of pittsburgh , pa . ( bxs - 318 ) hydrophobic silica ( treated with 1 pph silane coupling agent ) to form a pcm / silica dry powder . the silica was added to the liquid water / urea phase change material at room temperature , and a free flowing dry powder was obtained at a composition of water / urea / bxs 318 of 67 / 33 % by weight . thermal energy storage was determined by differential scanning calorimetry , at a heating and cooling range of 2 ° c ./ minute . in contrast to the sample of similar water / urea / silica dry powder made with cabot ms - 7 , the pcm / silica dry powder made with the ppg bxs 318 had a higher freezing temperature such as would be within the capacity of the ordinary freezers , or the freezer compartment on a household refrigerator . specific data computed from the dsc analysis were : table 5______________________________________ δhcmaterial tm ° c . tc ° c . tm - tc ° c . δhf cal / gm cal / gm______________________________________water / urea - 12 . 61 - 17 . 52 4 . 91 39 . 02 37 . 56 ( 10 / 1 ) bxs - 318water / urea - 12 . 45 - 17 . 38 4 . 91 38 . 04 36 . 64 ( 67 . 8 / 32 . 2 ) ______________________________________ instant cold packs , both of the prior art type , and of the type demonstrating the present invention were prepared as set forth in table 6 . those prepared in accordance with the present invention , i . e . utilizing a silica based gel or dry powder , were prepared using hydrophilic precipitated silica particles from ppg industries of pittsburgh , pa . ( referred to as silica abs or abs silica ) having a surface area of 150 m 2 / gram and an ultimate particle size of about 0 . 022 microns . in table 6 , samples 17 and 18 show the prior art form of cold packs based on ammonium nitrate and water . sample 18 contains seven parts of sodium chloride as a freezing point depressant . if salt were to be used , the concentration would need to be further increased ( as in the commercial products ) to prevent freezing to a hard lump of ice in reuse . samples 19 and 20 both contain abs silica sufficient to form a dry powder . sample 20 dry powder contains additionally seven parts of nacl freezing point depressant which is , in some respects , actually undesirable since it will make recharging by freezing , and reuse as a phase change material difficult . accordingly , the composition of sample 19 without salt is preferred . samples 21 and 22 of table 6 both contain sufficient abs silica to form a gel and the water used contains urea in the form of a water / urea ( 75 / 25 ) solution . this addition effectively prevents the gel from freezing to a hard ice at ordinary freezer temperatures and thus , provides reuse capability . an improved gel formulation is contained as sample 26 . in this case , the urea freezing point depressant is mixed directly into the ammonium nitrate prior to addition of the activating water . since the urea adds further endothermic heat , this formulation may be preferred since it will go to lower temperatures and does not require any premixing of the urea with water ( e . g . samples 21 and 22 ). the minimum temperature of sample 26 is about 20 ° f . the minimum temperature for the sample 21 without the urea added to the dry powder is about 28 ° f . accordingly , this example shows endothermic compositions comprising ammonium nitrate , urea , or combinations of ammonium nitrate and urea with and without a freezing point depressant ( salt or urea ) and water plus sufficient hydrophilic ( fumed or precipitated ) silica to form a reusable gel ( at lower silica concentrations ) or a reusable dry powder ( at higher concentrations ) when activated with water . table 6______________________________________instant cold packs based on ammonium nitrate / water for phase change laboratories one one dry dry use use powder powder gel gel gel 17 18 19 20 21 22 26______________________________________water ( gms ) 140 140 140 140 -- -- 140water / urea -- -- -- -- 187 187 --( 75 / 25 ) gms . urea gms . -- -- -- -- -- -- 47silica abs gms . -- -- 73 . 4 73 . 4 70 73 . 4 70water / silica % -- -- 65 / 35 65 / 35 72 . 7 / 71 . 9 / 72 / 28 27 . 3 28 . 1nh . sub . 4 no . sub . 3 gms . 168 . sup . ( 1 ) 168 . sup . ( 1 ) 168 . sup . ( 2 ) 168 . sup . ( 2 ) 168 . sup . ( 2 ) 168 . sup . ( 2 ) 168 . sup . ( 2 ) salt ( gms ) -- 7 -- 7 -- -- -- ______________________________________ . sup . ( 1 ) ammonium nitrate prills ( pellets ) . sup . ( 2 ) ammonium nitrate powder instant hot packs demonstrating the present invention were made as set forth in table 7 . the silica abs is the same as that discussed in example 4 . the two hot packs include duplicates of a gel ( no . 21 ) and dry powder ( no . 23 ) respectively based on calcium chloride and abs silica with water to activate . the gels give a higher maximum temperature , but the dry powders have better conformability . maximum temperatures in the range of 155 ° to 145 ° f . were recorded for the gel and dry powder respectively . accordingly , this example shows a chemical composition for &# 34 ; instant hot &# 34 ; applications comprised of a high exothermic molal heat of solution compound having a relatively low molecular weight , a high solubility in water , plus hydrophilic silica in an amount sufficient to form a gel or a dry powder when subsequently mixed with separately contained water in ratios determined by the solubility of the exothermic compound in water . table 7______________________________________instant hot pack based on calcium chloride / water / silica sample number ( 21 ) ( 23 ) sample type hot pack hot pack ( gel or dry powder ) gel dry powder______________________________________1 . water , distilled ( gms ) 200 2002 . ammonium nitrate ( gms ) 3 . calcium chloride ( gms ) 119 1194 . silica abs ( gms ) 53 98 . 55 . water / silica (%) 79 / 21 67 / 336 . min . or max . temp . ° f . 155 147______________________________________ the primary advantage is that the instant cold or instant hot reaction can be initiated by mixing with water anytime , anyplace , and the need for &# 34 ; charging &# 34 ; by prefreezing the instant cold or microwave heating the instant hot system is eliminated altogether . the primary disadvantage of the instant hot and instant cold gels and dry powders is that they cannot supply cold or heat for as long a time period as the comparable phase change materials , and the instant cold or hot is provided on a thermocline of ascending or descending temperature -- not on a plateau of constant temperature as in the comparable phase change materials . while certain representative embodiments and details have been shown for purposes of illustrating the invention , it will be apparent to those skilled in the art that various changes in the methods and apparatus disclosed herein may be made without departing from the scope of the invention ., which is defined in the appended claims .