216 Dr. Thomson on [Sept. 



strengths of these spmts, reckonuig from the degree of cold 

 induced by the evaporation of water. 



Having made three very dehcate hygrometers, according to 

 Leslie's construction, I moistened the bulb of one of them with 

 strong whisky, the bulb of another with a mixture of equal 

 quantities of the same spirits and water, and the bulb of the 

 tuird v.ith water. I watched the descent of the fluid in the 

 stem till each had gained its maximum of cold, and marked the 

 cold induced by the water 40, by the dilute spirits 64, and by 

 the strong spirits 88. Kovv the difference between 40 and 64 is 

 24, and between 40 and 88 is 48. Hence the following propor- 

 tion : 24 : 48 :: strength of the dilute, : strength of the strong 

 spirits. This I have tried with ditt'erent proportions of spirits 

 and water in different states of the atmosphere, and found the 

 same property uniformly obtain. The experiment requires to be 

 performed with great delicacy and care, as the spirits soon 

 acquire their maximum after which the fluid in the stem begins 

 to ascend. 



This curious property may, perhaps, be explained by consider- 

 ing the composition of the evaporating fluids. In the case of the 

 dilute spirits the evaporating film consists of an equal number of 

 aqueous and spirituous particles. Now if the cold induced by a 

 complete film of v/ater be 40, the cold produced by the aqueous 

 particles in the dilute film will be 20. Again, if the cold induced 

 by the evaporation of a complete film of spirits be 88, the cold 

 iiiduced by the spirituous particles in the dilute surface will be 

 44. Hence the cold induced by the evaporation of both will be 

 the sum of 20 and 44, or 64, agreeably to the actual experiment. 

 Leslie's differential thermometer, therefore, besides the various 

 uses to which that ingenious and profound philosopher has 

 applied it, may also be used instead of the hydrometer to deter- 

 mine the strength or specific gravity of spirits. Considering the 

 delicate structure of the instrument, and the difficulty of using 

 it, we do not expect, however, that it will ever supersede the 

 use of that invaluable instrument. 



Article IX. 



On Ferrochynzate of Iron. By Thomas Thomson, M.D. F.R.S. 



In a former number of the Annals of Pliilosoph/ I gave an 

 account of some ex]ieriments which 1 had made to determine 

 the composition of the beautiful blue pigment formed by pouring 

 a solution of ferrochyazate of potash, or of ammonia, into the 

 persulphate, pernitrate, permuriate, or any soluble ferruginous 

 salt, containing the peroxide of iron. These experiments satis- 

 fied me, as I trust they would satisfy the readers of the Annals, 



