ON THE SOLUBILITY OF SALTS. 307 



and apply a constant pressure during the whole time of the experiment. I 

 propose, in addition to diaphragms composed of capillary glass tubes cemented 

 together, to use also a series of platinum plates of various thicknesses, 

 and pierced with holes of successively increasing diameters. By means of 

 this instrument I shall be able not only to make experiments on the flow of 

 liquids from a full vessel through capillary tubes into a space filled only with 

 air, but likewise on the phenomenon of diffusion through such tubes — a kind 

 of osmosis corresponding to what Fick calls pore diffusion. 



Phenomena connected with Density. — The phenomena belonging to this 

 category which may serve as tests of molecular changes in saline solutions, 

 are, — 1, density at various temperatures of solutions saturated at 0° and 100° 

 and other intermediate points; 2, amount of condensation which takes place 

 in saturated solutions when effected at different temperatures ; 3, condensation 

 which takes place on diluting saturated solutions with successive equivalent 

 quantities of water ; 4, density of supersaturated solutions at various tempera- 

 tures ; 5, point of maximum density of solutions ; 6, influence of pressure 

 upon the point of maximum density, and on the amount of condensation 

 which results from dilution. The densities of a great number of saline solu- 

 tions have recently been determined with great care by Kremers and Gerlach, 

 and Ldwel. These furnish valuable data both for scientific and practical 

 purposes, and will be of the greatest assistance to me. In subjects of this 

 kind, both on account of their extent and character, every observer has 

 necessarily his own point of view, and makes use of different means to 

 attain the same result. Thus for my special object I consider it better to 

 confine my attention at first to saturated solutions, than to experiment upon 

 solutions of different degrees of concentration. With regard to the point of 

 maximum density, I shall only have to endeavour to continue what Despretz 

 has so admirably begun. 



Thermology of Saline Solutions. — I shall do little more than indicate on 

 this occasion the phenomena connected with heat, which may be studied as 

 tests of molecular changes. They are, — 1, expansion of saturated saline 

 solutions, already included under the head of density ; 2, contraction of 

 saturated solutions before they yield any crystals; 3, changes of volume 

 which accompany crystallization ; 4, point of congelation of saturated saline 

 solutions; 5, boiling-point of saturated solutions; 6, latent solution and 

 dilution heat of single salts, and of mixtures made according to the schemes 

 laid down above; 7, specific heat of such solutions; 8, tension of vapours 

 from saline solutions. Nothing need be said here on the subject of expan- 

 sion of saline solutions, as the value of determinations of the rate of ex- 

 pansion can only be judged of as tests of molecular changes when we are in 

 possession of data upon the other physical properties of solutions. The 

 contraction which saline solutions undergo in cooling before crystals separate 

 is of great importance, and in many cases will be found not to correspond 

 to the expansion when heated through the same number of degrees. The 

 changes of volume which accompany crystallization, and the point of con- 

 gelation of saline solutions, are intimately connected with the subject of their 

 latent solution and dilution heat and their specific heat. The study of these 

 phenomena includes, — 1, the determination of the specific heat of the solid 

 and fused anhydrous salt itself, and of the hydrates which it forms; 2, simi- 

 lar determinations for mixtures of salts according to the schemes laid down 

 above, whenever such mixtures can be fused together ; 3, and lastly, of 

 determinations of the latent fusion heat of simple salts and mixtures of salts. 

 As regards the boiling-points of saline solutions, I propose to divide this part 

 of the investigation into two parts; first, to determine the maximum effect 



x 2 



