WITH THE MICROSCOPE. 179 



As far as I am aware no satisfactory explanation has been given of 

 this curious phenomenon, and I can only suggest that it is in some 

 way related to those molecular movements on which sensible heat 

 appears to depend. 



2. On Making Sections of Rocks and Crystals. Compara- 

 tively little can be learned of the structure of rocks and minerals from 

 the examination of fractured surfaces by reflected light. Flat 

 polished surfaces show very much more, but nearly all the important 

 facts can only be observed by examining thin sections by trans- 

 mitted light. What is really requisite is. to have portions sufficiently 

 thin, flat, and smooth to transmit light. In some cases fragments of 

 clear minerals may be broken thin and flat enough to show certain 

 facts very well, when mounted in Canada balsam; and in this manner 

 we may easily study the fluid-cavities in quartz, or the structure of 

 such rocks as obsidian and pitch-stone. In many cases, however, we 

 must have recourse to carefully prepared thin sections. The details 

 of the method of preparing these must necessarily vary according to 

 the mechanical means at the disposal of each person, and much time 

 may be saved by the use of machinery. I shall therefore give such 

 a general account as may be used by any one who has not machinery 

 at command, premising that it will be easy to modify it in detail, 

 according to the facilities which each may possess for employing 

 more expeditious methods. 



In collecting specimens for examination, I find it convenient to 

 break off portions from the rock as flat and thin as possible, so that 

 they may be ground down at once ; for otherwise it may be requisite 

 to saw off portions with a lapidary's wheel, or by means of a straight 

 toothless saw of sheet-iron with emery. Having made the speci- 

 men of a convenient size and form, with one side flat, this must 

 be ground down perfectly level and dressed off very smooth. I 

 usually avoid using any polishing powder, since, if it were to work 

 into cracks or cavities, it would be far more objectionable than any 

 slight want of polish, If we attempt to grind down the surface on 

 such a stone as should be used to finish off, very much time would 

 be lost, and it is therefore best to use a series of stones of increasing 

 fineness. I have generally used first fine emery on a plate of iron or 

 -/me, then a kind of stone known by marble workers as "Congleton ;" 

 after that a soft piece of Water-of-Ayr stone, and finally finish off on 

 a very hard and fine-grained piece of the same kind. However, since 

 it may be difficult to procure such stones, a flat slab of fine-grained 

 marble, or different kinds of slate may be used. What is wanted is to 

 finish off the surface so as to be free from scratches and almost 

 polished, with the hardest and the softest portions ground down to 



N 2 



