1164 
MR. G. F. RODWELL OR THE EFFECTS OF HEAT OR CERTAIR 
256 C., contraction commences feebly and under a very low coefficient at 284° C., and 
continues till a temperature of 30.9° C. has been attained, when rapid expansion sets 
in. The difference in the volume due to this contraction is very slight: thus, volume 
at 284° C. = T011804, and volume at 309° 0. when the contraction is finished = 
1*010805. 
18. The microscope, when applied to the examination of thin layers, also shows 
differences. PbI 2 .AgI in a thin layer is opaque owing to the presence of multitudes 
of small crystals. Cu 3 I 2 . Agl is transparent. It would therefore appear that the 
presence of the Cu 2 I 2 has prevented the iodide of silver from changing from the 
amorphous plastic condition to the crystalline condition: a change which, as above 
stated, is usually accomplished at 142° C. 
19. The researches of petrologists in connexion with the examination of thin slices 
of minerals under the microscope, have shown that if rocks be fused and suddenly 
cooled no structure is developed, while, if slowly cooled, they not only become 
crystalline, but may be built up entirely of crystals. Thus basalt perfectly fused 
and suddenly cooled in water forms a black glass, which exhibits no structure under 
the highest power. Less rapid cooling develops microliths or crystallites, and as the 
cooling is still further retarded the microliths group themselves into gem-like forms, 
into radial forms about a centre, into small definite crystals, and into large definite 
crystals. Thus the glassy basalt or tachylyte, passes into ordinary basalt; and the 
ordinary basalt, into the highly crystallised gabbro. Also it is noted that the specific 
gravity increases as the crystalline structure is more and more developed. Thus 
glassy basalt has a lower specific gravity than basalt, and the latter than gabbro. 
We are also very familiar with the difference in the size of the crystals of any 
substance which separates out from solution if the evaporation be rapid or slow. 
Now Agl above 142° O. corresponds to the glassy basalt. 
,, below „ „ gabbro. 
Or octohedral sulphur corresponds to the gabbro. 
„ plastic „ „ „ tachylyte. 
The crystals of felspar, quartz, &c., doubtless separate out from solution in the 
ground mass of the mineral, as a soluble salt separates from its solution in water. 
20. We have obviously to deal with— 
(a) The crystalline forces which tend to build up the substance into crystals. 
(b) The force of cohesion exerted by the crystals whether microliths or large 
crystals for each other. 
(c) The attraction subsisting between the surrounding mass or medium and the 
crystals. 
On the other hand we have the molecular motion tending to disunite them. 
