OF MICROSCOPIC OBJECTS. 117 



perfect fluidity of the mixture. The tin bath is, perhaps, most 

 convenient when made like a small shallow cistern ; but some 

 close it on the top to place the pots upon it, and alter the shape 

 to their own convenience. 



8. We will now inquire into some of the materials which are 

 needed in this operation ; the first of which is size. This sub- 

 stance is often used in the form of glue, but it must be of the 

 very best and most transparent kind. To make the liquid which 

 is to receive the colours for the usual mode of injecting, take of 

 this glue seven ounces, and pour upon it one quart of clean 

 water ; allow this to stand a few hours, and then boil gently until 

 it is thoroughly dissolved, stirring with a wooden or glass rod 

 during the process. Take all impurities from the surface, and 

 strain through flannel or other fine medium. The weather affects 

 this a little as to its stiffness when cold, but this must be counter- 

 acted by adding a little more glue if found too liquid. 



9. Instead of glue, gelatine is generally used, especially when 

 the work to be accomplished is of the finer kind. The propor- 

 tions are very different in this case, one ounce of gelatine to about 

 fourteen ounces of water being sufficient. This, like glue, must 

 be soaked a few hours in a small part of the cold water, the 

 remainder being boiled and added, when it must be stirred until 

 dissolved. A good size may be made by boiling clean strips of 

 parchment for awhile, and then straining the liquid whilst hot 

 through flannel ; but when the injections are to be transparent, it 

 is of the greatest importance that the size be as colourless as 

 possible. For this purpose good gelatine must be employed, as 

 Nelson's or Cox's : some persons of experience prefer the latter. 



10. Colours. The size-solution above mentioned will need 

 some colouring matter to render it visible when injected into the 

 vessels of any animal, and different colours are used when two or 

 more kinds of vessels are so treated, in order that each " set " 

 may be easily distinguished by sight. The proportion in which 

 these colours are added to the size-solution may be given as fol- 



ows : 



