VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 101 



On the phenomena observed on mixing snow with these acids. — On Dec. 13, 

 snow was added to the spirit of nitre N° ]68. The snow was put in very gra- 

 dually, and time was taken to find what effect each addition had on the thermo 

 meter and mixture, before more was added. The temperature of the acid before 

 the mixture was — 27°, and each addition of snow raised the thermometer a 

 little, till it rose to — l°i; after which the next addition made it sink to — 2°; 

 which showed that sufficient snow had then been added. The quantity of 

 snow used was pretty exactly -^ of the weight of the acid, the weight of the 

 acid being 13 oz. so that the strength of the diluted acid was reduced to .411. 

 The acid before the addition of snow had no signs of freezing, its tempera- 

 ture being in all probability much above its freezing point; yet the snow did 

 not appear to dissolve, but formed thin white cakes, which however did not float 

 on the surface, but fell to the bottom, and when broken by the spatula formed 

 a gritty sediment ; so that it appears, that these cakes are not simply undissolved 

 snow, bat that the adjoining acid absorbed so much of the snow in contact with 

 it, as to become diluted sufficiently to freeze with that degree of cold, and then 

 congealed into these cakes. The quantity of congealed matter seems to have 

 kept increasing till the end of the experiment. 



On Dec. 21, an experiment was made in the same manner with the dephlo- 

 gisticated spirit of nitre N° 27. The acid began to freeze in pouring it into the 

 jar in which the mixture was to be made, and stood stationary there at — 19°; 

 so that the liquor at the beginning of the experiment was white and thick, which 

 made the effect of the addition of the snow less sensible. However the con- 

 gealed matter constantly subsided to the bottom, and the quantity seems to have 

 continued increasing to the end of the experiment. The heat of the mixture 

 rose to — 4° before cold began to be produced, and the quantity of snow added 

 was -^^ of that of the acid, so that the strength of the acid was reduced to 

 .437 by the dilution. 



A very remarkable circumstance in this experiment is, that the acid, while 

 the snow was adding, first became of a yellowish, and afterwards of a greenish 

 or bluish hue. This colour did not go off by standing, but continued at least 

 10 days, during which time the acid constantly kept that colour, except when 

 by hasty freezing it shot into small filaments, in which case it put on the white 

 appearance which these acids always assumed under those circumstances ; but 

 once that by gradual freezing it shot into transparent ice, which was of a bluish 

 colour. 



It is remarkable, that in both these experiments the addition of snow pro- 

 duced heat, till it arrived pretty exactly at what was found to be the freezing 

 point of the diluted acid ; but that as soon as it arrived at that point, the addi- 

 tion of more snow began to produce cold. This can hardly be owing merely to 



