133 



taking a drop of a warm saturated solution of the salt required, and 

 when a good group of well-defined crystals has been obtained, to 

 break through the marginal ring of crystalline deposit with a small 

 point of wood, and carefully conduct off the uncrystallized portion or 

 mother liquor to the extremity of the slide, at the same time placing 

 it in a vertical position to drain until it is dry. A small quantity of 

 castor oil is to be next carefully dropped on the subject and guided 

 over the field with the point of a needle ; in this way it readily 

 displaces the air and occupies the most minute cavities. After a short 

 time the upper glass is to 'be placed on the surface, taking care 

 to lower it gradually so as to exclude the air ; if the field is too full 

 of oil the excess may be removed by a small piece of bibulous paper, 

 and if, on the contrary, sufficient oil has not been used, an additional 

 portion can be readily introduced, by the capillary action, between 

 the glasses. I mention these points of practical manipulation because 

 on them the success of the operation may often depend, as it must be 

 evident that in the first case, the excess would prevent the cell from 

 being properly sealed by the varnish, and in the latter, it would 

 be drawn into the field and the whole operation spoiled. The shell-lac 

 varnish is then to be used as the cementing medium in the same way 

 as has been described, and with the same precautions. I may also 

 observe, that this varnish cannot be replaced by either of the others, 

 as it is actually necessary (and this should always be borne in mind) 

 that there should exist no affinity between the fluid in the cell and the 

 varnish used to seal it permanently. Hundreds of excellent objects 

 have been lost from this cause, and much valuable time and labour 

 thrown away. 



The reason why oil was selected for this purpose arose from its want 

 of action on most crystalline salts, many of which could not be pre- 

 served uninjured in any other medium. Castor oil was chosen 

 in consequence of its not depositing a crystalline stearine by reduc- 

 tion of temperature, as is the case with most oils, and even in some 

 specimens of this material, it therefore requires to be carefully ex- 

 amined in this respect before it is employed. 



In the same year I used this medium as a mounting fluid for minute 

 Fungi and Pediculi, with complete success, and exhibited them to 

 numerous microscopic friends, by some of whom it has also been 

 since employed with entire satisfaction. 



