9^ PHILOSOPHICAL TKANSACTIONS. [aNNO I786. 



with their balls immersed in a glass vessel filled with diluted oil of vitriol. They 

 were at times also compared in cold more violent than the natural cold of the 

 climate, by adding snow to the acid in which they were tried, in which case 

 care was taken to keep the mixture frequently stirred. Oil of vitriol was recom- 

 mended for this purpose, as a fluid which would most likely bear any degree of 

 cold without freezing, and whose natural cold might be much increased by the 

 addition of snow. It seems to have answered the purpose very well, and not to 

 have been attended with any inconvenience. 



During the first comparison of these thermometers, a whitish globule, such 

 as those which appear in frozen oil, was observed in the tube of the thermometer 

 filled with oil of sassafras. This appearance of congelation did not much in- 

 crease ; but 2 days after a large air bubble was found in its ball, which prevented 

 Mr. M'Nab from making further observations with it. It is well known, that 

 spirit of wine expands more by a given number of degrees of a mercurial ther- 

 mometer in warm temperatures than in cold ones ; and this inequality, as might 

 be expected, was less in the stronger spirit than in the weaker, but the difference 

 was inconsiderable. The oil of sassafras also had some of this inequality, but 

 much less. It however appears to be by no means a proper fluid for filling ther- 

 mometers with. No appearance was observed which indicates any considerable 

 irregularity in the contraction of spirits of wine in intense cold, or which renders 

 it probable, that thermometers filled with it could be sunk by a mixture of snow 

 and spirit of nitre to a degree near approaching to that mentioned by Professor 

 Braun. 



Mr. M'Nab in his experiments sometimes used one thermometer and some- 

 times another ; but in the following pages I have reduced all the observations to 

 the same standard ; namely, in degrees of cold less than that of freezing mer- 

 cury I have set down that degree which would have been shown by the mercurial 

 thermometer in the same circumstances ; but as that could not have been done in 

 greater degrees of cold, as the mercurial thermometer then becomes of no use, 

 I found how much lower the mercurial thermometer stood at its freezing point, 

 than each of the spirit thermometers, and increased the cold shown by the latter 

 by that difference. 



On the Common and Dephlogisticated Acids of Nitre. — Many experiments 

 show, that both these acids are capable of a kind of congelation, in which the 

 whole, and not merely the watery part, freezes. Their freezing point also 

 differs greatly according to the strength, and varies according to a very unex- 

 pected law. Like water too they bear being cooled very much below their 

 freezing point before the congelation begins, and as soon as that takes place, 

 immediately rise up to the freezing point. The white colour of the ice in these 

 experiments seems owing only to its consisting of very slender filaments ; for in 



