274 THE AMERICAN MONTHLY [December, 



The thickness of the sections is regulated by a thumb screw at its base. 

 The paraffine plug, containing the embedded material to be cut, is 

 made to protrude from the upper end by the turning of a turn of the 

 screw. The razor is supported on a circular plate, which surrounds 

 the top, and is kept wet with methylated alcohol. An advantage which 

 this instrument supplies is this : if it is desirable to examine the first 

 few sections before proceeding to cut the others, the microtome and its 

 contents may be inverted in spirit and kept moist till it is wanted. It 

 also furnishes a ready means of cutting sections from stems of plants. 

 Hardwood sections are best obtained from the shavings, which may be 

 procured from any cabinet-maker. 



In this way, sections sufficiently thin to show the structure well, may 

 be procured. Good longitudinal sections of hardwood, as walnut, teak, 

 mahogany, and cedar, when mounted in Canada balsam, make inter- 

 esting slides. Fossil woods being silicified, can only be cut by a lapi- 

 dary's wheel. These and sections of coal which may be placed under 

 the head of vegetable sections, are often purchased from dealers, for coal 

 cutting is dirty work, and troublesome. 



The animal tissues in their recent condition are too soft to admit of 

 thin sections being cut. They have therefore to go through a prelimi- 

 nary hardening process. There are various hardening agents with 

 respective capabilities and advantages. One of the readiest is absolute 

 alcohol. Being anhydrous, it soon abstracts all water from a tissue, 

 and hardens it ready for cutting in about twenty-four hours ; but its ac- 

 tion is so powerful that, while it hardens rapidly, it causes an undue 

 shrinkage, and may mislead our. ideas of the nature and histological 

 character of a section. It is better that the tissue should be immersed 

 in a large quantity of some aqueous solution containing chemicals, 

 which has the power of coagulating its albuminous element. There 

 are a number from which to choose. A two to five per cent, solution 

 of bicromate of ammonium, or of potash, used in considerable bulk, 

 compared with the size of the mass of the tissue to be hardened, works 

 as satisfactorily as any. Chromic acid, in strengths of one-fifth per 

 cent, to one-half per cent., ma}' also be recommended ; but if all risk of 

 shrinkage is to be avoided, it is better to begin with a weak solution, 

 and gradually increase its strength. Muller's solution is also valuable 

 in many cases. 



It is made by adding two and a half parts of bichromate of potash 

 and one part of sulphate of soda to one hundred parts of water ; but, 

 unless the student intends extended histological research, the ammonium 

 bichromate will be sufficient. 



In whatever manner sections are cut, one thing is necessary, and that 

 is, the possession of a very sharp, hollow-ground razor. Great care 

 must be taken that its keenness is maintained by frequent strapping on 

 a firm flat strap. Sections will be obtained with greater success if it 

 is strapped after every two or three cuts. When finished with for the 

 day, it should be strapped again and put away clean. Many are very 

 careless in this respect. Do not let it get dirty and spotted with rust 

 when it ought to be so smooth that the sections it cuts should slip 

 over its surface freely. If vSpots of rust are allowed to roughen this sur- 

 face, the sections will be caught in them and torn. 



The next step in section cutting is embedding. The hardened tissue, 



