9. Instead of glue, gelatine is generally used, especially when the work to be accomplished is of the finer kind. The proportions 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 follows:-- 11. For-- Red 8 parts of size-solution (by weight) to 1 part of vermilion. Yellow 6 ” ” 1 ” chrome yellow. White 5 ” ” 1 ” flake-white. Blue 3 ” ” 1 ” blue-smalt, fine. Black 12 ” ” 1 ” lamp-black. Whichever of these colours is made use of must be levigated in a mortar with the addition of a very small quantity of water until every lump of colour or foreign matter is reduced to the finest state possible, otherwise in the process of injecting it will most likely be found that some of the small channels have been closed and the progress of the liquid stopped. When this fineness of particles is attained, warmth sufficient to render the size quite fluid must be used, and the colour added gradually, stirring all the time with a rod. It may be here mentioned that where one colour only is required, vermilion is, perhaps, the best; and blue is seldom used for opaque objects, as it reflects very little more light than black.