28 o 
Mr. Walker's Observations on the 
At a higher temperature than 50°, the quantity of the salts 
must be increased, and the effect will be proportionably greater; 
at a lower temperature diminished, when the effect will be pro- 
portionably less. 
It must be observed, that to produce the greatest effect by 
any frigorific mixture, the salts should be fresh crystallized,* 
not damp, and newly reduced to very fine power ; the vessel 
in which they are made very thin, and just large enough to 
contain the mixture ; and the materials mixed intimately to- 
gether, as quickly as possible, the proper proportions at any 
temperature (those in the Table being adjusted for the tem- 
perature of 50° only ) having been previously tried, by adding 
the powdered salts gradually to the liquid, till the thermo- 
meter ceased to sink ; observing to produce the full effect of 
one salt before a second is added, and likewise of the second 
before a third is added. Neither soda, phosphorated soda, nor 
Glauber's salt should be mixed with nitrous ammoniac, or 
the powder composed of sal ammoniac and nitre, unless at a 
low temperature, i. e. below o°, but pounded and kept apart. 
In the experiments alluded to in the Table, the precaution 
of fresh crystallizing the salts was not observed, because I 
chose to give the ordinary effects only ; I therefore then used 
salts in their common state, taking care, however, to choose 
such as had not in the least effloresced. 
Since it is always useful, and generally absolutely necessary, 
by weight, well agitated together, and become cool : d. vitr. acid, is strong vitriolic 
acid, and rain, or distilled water, equal parts, by weight, thoroughly mixed (very 
cautiously) and cooled. 
* Soda, phosphorated soda, and Glauber’s salt, are best crystallized afresh,. be- 
cause their effect, especially the two last in the acids, depends upon the quantity of 
water they contain in a solid state. 
