464 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the glass. Softer rocks may require gentle treatment with the Water- 

 of-Ayr stone. 



The last parts of the process are the most delicate of all. We 

 desire to make the section as thin as i:)ossible, and for that purpose 

 continue rubbing until after one final attempt we perhaps find to our 

 dismay that great part of the slice has disappeared. The utmost 

 caution must consequently be used. The slide should be kept as flat 

 as possible, and looked at frequently, that the first indications of dis- 

 ruption may be detected. The thinness desirable or attainable depends 

 in great measure upon the nature of the rock. Transparent minerals 

 need not be so much reduced as more opaque ones. Some minerals, 

 indeed, remain absolutely opaque to the last, like i>yrite, magnetite, 

 and ilmenite. 



The slide is now ready for the Microscope. It ought always to be 

 examined with that instrument at this stage. We can thus see whether 

 it is thin enough, and if any chemical tests are required they can 

 readily be applied to the exposed surface of the slice. If the rock has 

 proved to be very brittle, and we have only succeeded in procuring a 

 thin slice after much labour and several failures, nothing further 

 should be done with the preparation unless to cover it with glass, as 

 will be immediately exj^lained, which not only protects it, but adds to 

 its transparency. But where the slice is not so fragile, and will bear 

 removal from its original rough scratched piece of glass, it should be 

 transferred to one of the glass slides (No. 10). For this purpose the 

 prejmration is once more placed on the warm iron plate, and close 

 alongside of it is put the glass slide, which has been carefully cleaned, 

 and on the middle of which a little Canada balt^am has been dropped. 

 The heat gradually loosens the cohesion of the slide, which is then 

 very gently pushed along to the contiguous clean slip of glass. Con- 

 siderable practice is needed in this part of the work, as the slice, 

 being so thin, is apt to go to pieces in being transferred. A gentle 

 inclination of the warm plate is advantageous, so that a tendency may 

 be given to the slice to slip downwards of itself on to the clean glass. 

 We must never attempt to lift the slice. All shiftings of its position 

 should be performed with a point of a long needle or other sharp in- 

 strument. If it goes to pieces, we may yet be able to pilot the frag- 

 ments to their resting-place on the balsam of the new glass, and the 

 resulting slide may be sufficient fur the required purpose. 



When the slice has been safely conducted to the centre of the glass 

 slip, we put a little Canada balsam over it, and allow it to be warmed 

 as before. Then taking with the forceps one of the well-cleaned thin 

 cover-glasses, we allow it gradually to rest ujion the slice by letting 

 down first one side, and then by degrees the whole. A few gentle 

 circular movements of the cover-glass with the point of the needle or 

 the forceps may be needed to insure the total disappearance of air- 

 bubbles. When these do not aj^pear, and when, as before, we find 

 that the balsam has acquired the proper degree of consistence, the 

 slide containing the slice is removed, and jjlaced on the table with a 

 small lead weight above it in the same way as already described. On 

 becoming quite cold and hard the superabundant balsam round the 



