WITH THE MICROSCOPE. 53 



pletely covers the cell, and a certain quantity of the superfluous fluid 

 is pressed out. By the aid of any pointed instrument a very little 

 cement is removed from one part, so that more fluid may escape as 

 the cover is pressed down gently into the cement. The pressure 

 must be removed very gradually, or air, of course, will enter through 

 the hole. A bubble of air entering in this manner may often be 

 expelled again by pressure, or it may be driven out by forcing in 

 more fluid through a very fine syringe at another part of the cell ; 

 but it is far better to prevent the entrance of air in the first instance. 

 The edge of the glass cover being thoroughly embedded in the 

 cement, the small hole is to be carefully plugged up with a small 

 piece of cement, and the cell allowed to stand perfectly still for a 

 short time, when it may be very gently wiped with a soft cloth. 

 The edges of the cement may be smoothed by the application of a 

 warm iron wire, and any superabundance removed with a sharp 

 knife. A little Brunswick black or other liquid cement may be 

 applied to the edges, for the purpose of giving the whole a neater 

 appearance. 



The cement is made as follows : A certain quantity of India- 

 rubber scraps is carefully melted over a slow fire in a covered iron 

 pot. The mass must not be permitted to catch light. When it is 

 quite ,fluid, lime, in a perfectly fine powder, having been slacked by 

 exposure to the air, is to be added by small quantities at a time, 

 the mixture being well stirred. When moderately thick, it is removed 

 from the fire and well beaten in a mortar and moulded in the hands 

 until of the consistency of putty. It may be coloured by the addition 

 of vermilion or other colouring matter. I have several preparations 

 which have been placed in the creosote and naphtha solution in large 

 cells, and they are now perfectly air-tight, although upwards of twenty 

 years have elapsed since they were first put up, The lime and 

 India-rubber cement answers well for fixing on the glass tops of 

 large preparation jars, and looks very neat ; but, if moderately 

 strong spirit be used, a little air must be permitted to remain in 

 the jar. 



PRESERVATIVE FLUIDS. 



In all cases an object to be mounted in a preservative fluid should 

 be soaked in a considerable quantity of it for at least a day before 

 it is mounted permanently, and if the specimen is large, it should be 

 soaked for many days previous to being finally placed in the cell. 



99. Spirit and Water. Spirit and water form a well-known 

 and valuable medium for preserving anatomical preparations. In 

 uPuting spirit, distilled water only should be employed ; for if common 



