70 
Sir Humphry Davy on the 
and of a red, inclined to purple, in that of the oil ; and in 
both cases permanent elastic fluid was produced by its trans- 
mission. 
The law of the diminution of the density of vapours by 
diminution of temperature, has not been accurately ascer- 
tained ; but I have no doubt, from the experiments of Mr. 
Dalton, and some I have made myself, that it is repre- 
sented by a geometrical progression ; the decrements of tem- 
perature being in arithmetical progression ; and in three 
pure fluids that I operated upon,* the ratio seemed nearly 
uniform for the same number of degrees below the boiling 
point; and (taking intervals of 20 degrees of temperature) 
•3 694 * 16 . Upon this datum, and considering the boiling points 
of mercury to be 6oo°, that of oil 540°, that of the chloride of 
antimony 340°, and that of tin 5000° all above 52 0 , and the 
elastic force of vapour of water at this temperature to be 
equal to raise by its pressure about .45 parts of an inch of 
mercury ; the relative strengths of vapour will be, for mer- 
cury 000015615, for oil 0016819, for chloride of antimony 
01692, and for tin 37015, preceded by 48 zeros. -f 
It is not known whether the vapour from solids follows a 
similar law of progression as that from fluids, and these 
numbers are only given to show how minute the quantity of 
matter must be in vapours where its effects are distinct upon 
electrical phenomena ; and how much more minute it must 
be in the case of mercury artificially cooled ; and almost be- 
yond imagination so in vapours from substances requiring 
very elevated temperatures for their ebullition. 
* Water, chloride of phosphorus, and alcohol or carburet of sulphur. 
f I am obliged to Charles Babbage, Esq. F. R. S. for these calculations. 
