238 THE MICROSCOPE. 



be alcohol that is employed. It is better that any and 

 every solution should be too iveak than too strong; 

 for in the latter case the tissue is liable to become 

 friable and break down under the cutting knife or 

 razor, and the sections will not take the stain evenly. 

 To recapitulate and enforce one or two points of impor- 

 tance, note that the most useful strength for chromic 

 acid is a | or i per cent, solution ; for the bichromate- 

 of potash or ammonia 1% or 2 per cent, solution. When 

 either of the latter solutions are used the material 

 must be removed to an alcoholic solution in a week, or 

 ten days at most, or it will become very brittle. Alco- 

 hol is one of the best fluids for those to use who have 

 not much time to devote to the subject ; tissues har- 

 dened in alcohol afford, as a rule, the best staining 

 results. The price of alcohol is much against its use,, 

 but it may be used weak at first, and gradually increased 

 in strength until the material is found hard enough to 

 fcut with a knife. For cutting sections by hand the 

 best substance for embedding is a mixture of equal 

 / parts of olive oil and white wax ; or Cacao butter, or 

 ( even soap dissolved in alcohol (Micros. Journal, vol. ii. 

 \ p. 940, 1879) ; while in section-cutting machines with 

 "hollow cylinders, either the pith of elder, carrot, or 

 some other soft substance may be employed. If a razor 

 is used, its surface must be kept moist with water when 

 the freezing process is adopted, or with spirit w r hen 

 the hardening process without freezing is used. By 

 the freezing method we are enabled to cut and finish 

 off specimens sooner and more expeditiously than by 

 any other process. The material about to be frozen 

 must be removed from the hardening; fluid and well 

 washed in clean water before it is transferred to the 

 machine. Zeiss's microtome, with its surface of glass, 

 a practical and useful cutting and freezing machine, 

 can be obtained at a moderate price, of Baker, 

 Holborn. 



Swift's freezing microtome has the advantange of 

 preserving the preparation for some hours unchanged 

 in the frozen state. The method of using it is aa 

 follows : Remove the lid of the box and fill the cham- 



