the Point oj Congelation. 303 
elude, that greater cold could be produced with fnow by a 
mixture of falts than by means' of either taken feparately 
I made feveral experiments with this view, aq.d found it uni- 
formly the fa Q, that by adding a certain proportion of a fait 
which had lets power of producing cold with fnow, to one 
which had a greater power, the frigorific effect of the latter 
was fenfibrly incrcafed. Palling over examples of lefs confe- 
quence, it will be fufficient to inftance common fait and lal 
ammoniac. The ordinary common fait I ufed to mix with 
fn ow, funk the thermometer to — 5 0 ; the fal ammoniac to + 4 0 ; 
but when fome of the latter fait was mixed with the former, 
the compofition produced with fnow a cold of - 12 0 . I did 
not carry the experiments far enough to determine the propor- 
tions which anfwered heft, but faw evidently, that a confidera- 
ble variety in this refpedt occafioned fcarcely a perceptible, dif- 
ference : more fal ammoniac was, however, required, than 
that proportion of it to the common fait, which ought, by 
computation, to have funk a faturated. folution of the latter 7 
degrees. On feveral occafions 1 made ufe of this compofition 
to obtain a greater degree of cold than common fait alone 
would produce, and found it a very convenient method. 
On this principle it is that impure common fait always 
makes a ftronger freezing mixture than the pure; it being, in 
fa£t, a compofition of falts. I have feen three falts produce a 
greater cold than two, but have not carried the experiments 
far enough to afeertain the limits of this effedt *. 
M a . 
* The curious experiments, related in the Philofophicat Tranfa&ions for 
2787, p. 282. of producing a great degree of cold by diflolving a mixture 
of falts in water, depend in part upon the fame principle, the water being 
capable of reducing more fait from a folid to a fluid flate when feveral kinds are 
employed. 
