63 REPORT— 1875. 



•with only rare exceptions, between 1 and 1-3:1. The ratios of the con- 

 ductivities in the directions of these diameters, being the squares of the ratios 

 of the measured axes, range in value, for the majority of crystals, between 

 1 and 1'7: 1, and rarely approach the values above given for some of the 

 talcose and schistose rocks. It is shown in his researches that the principal 

 axes of conductivity in crystals are more closely related to the planes of 

 cleavage thaia to either the optic or the crystallographic axes, and that in 

 rocks of schistose structure it is to the internal texture arising from pressure 

 in metamorphic actions rather than to crystalline admixtures, giving to some 

 rocks a regularly streaked or veined appearance, that the principal develop- 

 ment of the property of unequal thermal conductivity in different directions 

 presented by this numerous class of rocks should be ascribed. These new 

 considerations, and the further recognition of the important part which the 

 saturation of certain rocks with water must exercise in determining the 

 distribution of underground temperature in certain cases, together with the 

 observation, recorded in the Table of this Eeport, of the extremely high con- 

 ductivity of quartz forming compact masses in the neighbourhood of i;nder- 

 ground workings, in some situations of considerable extent, are points of 

 special interest connected with the progress of this inquiry, the applications 

 and extensions of which, should the Committee pursue this iuquii-y further, 

 will form the chief object of their immediate investigations. 



Preliminary Report of the Committee, consisting of Professors Roscoe, 

 Balfour Stewart, and Thorpe^ apj)ointed for the purpose of ex- 

 tending the observations on the Specific Volumes of Liquids. Drawn 

 up by T. E. Thorpe. 



We are indebted to the experimental and critical labours of Hermann Kopp 

 for the greater part of what we know concerning the relations between the 

 specific gravities of liquids and their chemical composition. Kopp has pointed 

 out that when the specific volumes of liquids are compared at temperatures 

 at which their vapour-tensions are equal, as at their boiling-points, several 

 remarkable relations manifest themselves. In the first place, it is found that 

 the specific volume of a liquid formed by the union of two other liquids is 

 equal to the sum of the specific volumes of its components. Secondly, Kopp 

 finds that isomeric liquids of the same chemical type have identical specific 

 volumes. Thirdly, that in a series of homologues each increment of CH is 

 attended with a constant increment in specific volume. Hence Kopp was 

 able to assign certain fundamental values to a number of elementary bodies, 

 and thus to calculate with a considerable degree of accui-acy the specific 

 volume, and hence the specific gravity, of many liquid substances. It also 

 ajjpeared probable that members of the same family of elements have identi- 

 cal specific volumes, or, to use Schroder's expression, are " isosterous." Thus 

 the analogously constituted terchlorides of arsenic and phosjAorus appear to 

 possess the same specific volume ; whence it follows, since no change is ob- 

 servable in the volume occupied by chlorine in different compounds, that the 

 specific volumes of arsenic and phosphorus are equal. A similar conclusion 

 was drawn with respect to tin and titanium, members of the tetratomic group, 

 from an examination of their tetrachlorides. 



