5C4 CALCAREOUS SKELETON OF ECHIXODERMATA. 



(§§ 138-140). But their peculiar texture requires that certain 

 precautions should be taken ; in the first place, in order to prevent 

 the section from breaking whilst being reduced to the desirable 

 thinness ; and in the second, to prevent the interspaces of the net- 

 work from being clogged by the particles abraded in the reducing 

 process. — A section of the Shell, Spine, or other portion of the 

 Skeleton should first be cut with a fine saw, and be rubbed on a 

 flat file until it is about as thin as an ordinary card, after which it 

 should be smoothed on one side by friction with water on a Water- 

 of-Ayr stone. It should then be carefully dried, first on white 

 blotting-paper, afterwards by exposure for some time to a gentle 

 heat, so that no Water may be retained in the interstices of the 

 network, which would oppose the complete penetration of the 

 Balsam. Next, it is to be attached to a glass-slip by Balsam 

 hardened in the usual manner ; but particular care should be 

 taken, first, that the Balsam be brought to exactly the right 

 degree of hardness, and second, that there be enough not merely 

 to attach the specimen to the glass, but also to saturate its sub- 

 stance throughout. The right degree of hardness is that at which 

 the Cement can be with difficulty indented by the thumb-nail ; if 

 it be made harder than this, it is apt to chip-off the glass in 

 grinding, so that the specimen also breaks away ; and if it be 

 softer, it holds the abraded particles, so that the openings of the 

 network become clogged with them. If, when rubbed-down nearly 

 to the required thinness, the section appears to be uniform and 

 satisfactory throughout, the reduction may be completed without 

 displacing it ; but if (as often happens) some inequality in thick- 

 ness should be observable, or some minute Air-bubbles should 

 show themselves between the Glass and the under surface, it is 

 desirable to loosen the specimen by the application of just enough 

 heat to melt the Balsam (special care being taken to avoid the pro- 

 duction of fresh air-bubbles), and to turn it over so as to attach 

 the side last-polished to the Glass, taking care to remove or to 

 break with the needle-point any Air-bubbles that there may be in 

 the Balsam covering the part of the glass on which it is laid. The 

 surface now brought uppermost is then to be very carefully ground 

 down ; special care being taken to keep its thickness uniform 

 through every part (which may be even better judged -of by the 

 Touch than by the Eye), and to carry the reducing process far 

 enough, without carrying it too far. Until practice shall have 

 enabled the Operator to judge of this by passing his finger over 

 the specimen, he must have continual recourse to the Microscope 

 during the later stages of his work ; and he should bear constantly 

 in mind that, as the specimen will become much more transparent 

 when mounted in Balsam and covered with Glass, than it is when 

 the ground surface is exposed, he need not carry his reducing pro- 

 cess so far as to produce at once the entire transparence he aims 

 at the attempt to accomplish which would involve the risk of the 



