2 04 Dr. Blag den’s Experiments on 
M. de Reaumur propofed to try what cold gunpowder 
would produce with fnow, as a ted: of its goodnefs, that is, 
of the purity of the nitre in its compofition. It is evident, 
from what has been delivered above, that if any faline impu- 
rity be mixed with the nitre, a frigorific mixture formed with 
the compofition will be lo\yer than 26T5 a greater or lefs 
number of degrees, according to the nature and quantity of 
the impurity ; and if it be only common fait, the proportion 
of it to the nitre may be thus determined pretty nearly. 
As the cold produced by common fait with fnow is - 4 0 or 
more, and by fal ammoniac + 4 0 , it is difficult to conceive in 
what manner Fahrenheit fixed the zero of his thermometer. 
All thofe who have examined the few authentic paffages to 
be found in authors upon this fubjedt, will be fenfible how 
vaguely they are expreffed, leaving it doubtful whether he 
ufed common fait alone, fal ammoniac alone, or both mixed 
together. There is no method of finking a thermometer 
exadly to o° with thefe falts and fnow, but by means of a 
certain fmaller proportion of common fait added to the fal 
ammoniac ; and it would have been an extraordinary chance, 
that Fahrenheit ffiould hit precifely this proportion often 
enough to make him rely upon the point fo found as the 
commencement of his fcale ; more efpecially as the proportion 
employed, than it could of one of the kinds only. A remarkable fa< 5 t of this nature, 
as I imagine, is recorded fo early as in the Memoirs of the French Academy of 
Sciences for 17GO, p. 1 1 6. “On prend une livre de fublime corrofif, er une 
“ livre de fel ammoniac; on les puiverife chacun a part; on mele enfuite les 
“ deux poudres tres exa&ement ; on met le melange dans un matrns, verfant 
“ par-deffus trois chopines de vinaigre dift.il le ; on agite bien le tout: et ce nie- 
<l lange fe refroidit fi fort, qu'on a peine a tenir long-terns le vaiffeau dans les 
“ mains en ete. 11 eft arrive meme quelque fois a M. IIomberg, que faifant ce 
“ melauge en grande quantity, la matiere s’elt gele.” 
IS 
