Algebraic manipulation of normal, $\chi^2$ and Gamma probability distributions If $X_1, \ldots, X_n \sim N(\mu, \sigma^2)$, then 
$$
\frac{n - 1}{\sigma^2}S^2 \sim \chi^2_{n - 1}
$$ 
where $S^2 = \frac{1}{n-1}\sum_{i=1}^n (x_i^2- \bar{x})^2$, and there's a direct relationship between the $\chi^2_p$ and Gamma($\alpha, \beta$) distributions:
$$
\chi^2_{n - 1} = \text{Gamma}(\tfrac{n-1}{2}, 2).
$$
But then why is
$$
S^2 \sim \text{Gamma}(\tfrac{n-1}{2}, \tfrac{2\sigma^2}{n-1}) \,?
$$ 
And why do we multiply the reciprocal with $\beta$ and not $\alpha$?  Is it because $\beta$ is the scale parameter?  
In general, are there methods for algebraic manipulation around the "$\sim$" other than the standard transformation procedures?  Something that uses the properties of location/scale families perhaps?

Suppose $X\sim \operatorname{Gamma}(\alpha,\beta)$, so that the density is $cx^{\alpha-1} e^{-x/\beta}$ on $x>0$, and $\beta$ is the scale parameter.  Let $Y=kX$.  The density function of $Y$ is
$$
\frac{d}{dx} \Pr(Y\le x) = \frac{d}{dx}\Pr(kX\le x) = \frac{d}{dx} \Pr\left(X\le\frac x k\right) = \frac{d}{dx}\int_0^{x/k} cu^{\alpha-1} e^{-u/\beta} \, du
$$
$$
= c\left(\frac x k\right)^{\alpha-1} e^{-(x/k)/\beta} \cdot\frac1k
$$
$$
=(\text{constant})\cdot x^{\alpha-1} e^{-x/(k\beta)}.
$$
So it's a Gamma distribution with parameters $\alpha$ and $k\beta$.
