324 Tlie Philosophical Society of Glasgow. 



When different salts are dissolved in water, and the water gradually 

 evaporated, the salts wUl crystallize from the water separately and dis- 

 tinctly. Carbon and hydrogen will combine and form distinct and 

 differently constituted compounds according to the temperature they 

 have been submitted to. In the same way when we have a number of 

 bodies, as silica, alumina, Ume, iron, magnesia, &c., kept for a long time 

 in a fluid state by heat, and this heat is gradually withdrawn, analogous 

 to the slow evaporation of water, these substances wiU group themselves 

 together according to their affinities, which, \vith the simultaneous 

 action of the crystallizing force, while the compound is passing into the 

 soUd state, will form crystals large or small, according to circumstances. 

 From their specific gravity being little different from the fluid they are 

 formed in, these crystals will remain diffused. This supposition would 

 of course infer that all crystals forming porphyritic traps are less 

 fusible than the matrix in which they are imbedded, and from which 

 they have been formed ; and this is actually the case, so far as I have 

 been able to judge. This production of crystals within the trap form- 

 ing porphyiy must not be confounded with ordinary crystalline trap, 

 such as the hornblendic, where the whole mass is crystaUine, having 

 assumed that form in cooling, or rather setting, similar to what is often 

 found to take place in slag or scoria from smelting furnaces. 



Me. Geobge Blair described and experimentally illustrated some new 

 apparatus for measuring, weighing, and regulating the force of the 

 voltaic current. He placed the following instruments before the 

 Society : — 



1. A rheostat, constructed of tinned paper, ^-inch in breadth ; 

 resistance 1 inch = 400 feet of telegraph wire, No. 8. It gave resist- 

 ance without induction. 



2. A Gaugain's tangent galvanometer, with needle and shade and 

 stand in one piece. Ring = 9 inches in circumference ; six coils of 

 different sizes of wire, from No. 34 to No. 16. 



3. A galvanometer balance needle, lifting power = 44 grains ; is 

 turned with force of 50 grains in a powerful current. 



Mr. B. remarked that Kohbrausch had found sometimes the 

 negative, sometimes the positive electricity preponderate in three 

 batteries, when one pole was put to the eai'th ; and no doubt this arose 

 from the fact that the earth is not quite neutral. Say negative pole, 

 tension, 42; positive, 40; earth, 1. Hence there must be currents ; and 

 the existence of currents proves that the earth is not neutral. He had 

 found that sheet gutta percha is a better insulator than varnished 

 glass, unless the shellac is laid on when hot ; but when the glass is 

 quite dry and warm, it is the best of the two. 



