158 Royal Society : — Messrs. Dupre and Page on the Physical 



particle would be smaller than if the vapour were heavy and the 

 liquid light. There would evidently be more shrinkage in the one 

 case than in the other : these considerations were found valid through- 

 out the experiments. The case of toluol may be taken as representa- 

 tive of a great number of others. The specific gravity of this liquid 

 is 0*85, that of water being unity ; the specific gravity of its vapour 

 is 3*26, that of aqueous vapour being 0*6. Now, as the size of the 

 cloud-particle is directly proportional to the specific gravity of the 

 vapour, and inversely proportional to the specific gravity of the 

 liquid, an easy calculation proves that, assuming the size of the va- 

 pour polyhedra in both cases to be the same, the size of the particle 

 of toluol cloud must be more than six times that of the particle of 

 aqueous cloud. It is probably impossible to test this question with 

 numerical accuracy ; but the comparative coarseness of the toluol 

 cloud is strikingly manifest to the naked eye. The case is, as I have 

 said, representative. 



In fact, aqueous vapour is without a parallel in these particulars ; 

 it is not only the lightest of all vapours, in the common acceptation 

 of that term, but the lightest of all gases except hydrogen and am- 

 monia. To this circumstance the soft and tender beauty of the clouds 

 of our atmosphere is mainly to be ascribed. 



The sphericity of the cloud-particles may be immediately inferred 

 from their deportment under the luminous beams. The light which 

 they shed when spherical is continuous : but clouds may also be pre- 

 cipitated in solid flakes ; and then the incessant sparkling of the cloud 

 shows that its particles are plates, and not spheres. Some portions of 

 the same cloud may be composed of spherical particles, others of 

 flakes, the difference being at once manifested through the calmness 

 of the one portion of the cloud, and the uneasiness of the other. 

 The sparkling of such flakes reminded me of the plates of mica in 

 the river Rhone at its entrance into the Lake of Geneva, when shone 

 upon by a strong sun. 



March 1 1 . — Lieut.-General Sabine, President, in the Chair. 



The following communication was read :— 



" On the Specific Heat and other physical properties of Aqueous 

 Mixtures and Solutions." By A. Dupre, Ph.D., and F. J.M. Page. 



Part I. 



Mixtures of Ethylic Alcohol and Water. 



Section 1 . Specific Heat. 



For the methods employed in estimating the specific heat of these 

 mixtures, see a former abstract, ' Proceedings of the Royal Society,' 

 vol. xvi. p. 336 (Phil. Mag. S. 4. vol. xxxv. p. 464). 



r a the present paper the authors give the specific heat of an ad- 

 ditional number of mixtures, so as to complete the series for every 

 10 per cent, from water to absolute alcohol. 



The following Table gives the mean of the results obtained in all 

 experiments, details of seventy-four of which are given : — 



