The volatile oils being nearly equally penetrating, turpentine is generally chosen, because it is cheaper. Nevertheless, for small objects, the citron, or that of a species of lavender (aspie of the shops) is preferred, on account of their odour, which are besides not very expensive. When it is intended to inject only with one of these oils, which makes a liquid matter extremely penetrating, after having dissolved a colouring matter previously ground in a fixed oil, the mixture is slightly heated. This liquor is generally employed to render perceptible the small vessels of membranes, which are not to be dissected, but well preserved in their integrity. If it is intended to inject the large trunk which supplies these membranes towards the end of the operation, it is necessary to inject a little essence of varnish, charged with much resin, and before drying the piece, let it soak a day or two in an aqueous solution of the deuto-chloride of mercury, according to the process of Chaussier.

The matters with which the volatile oils are to be coloured, should be previously ground with the greatest care. It is easy to procure those which are prepared with nut oil, and which are sold in little bladders to be employed upon palettes.

Colours, thus prepared and intimately amalgamated with fixed oils, remain much better suspended; the heaviest oxides, even those of lead and mercury, are not then subject to become deposited.

Resins, dissolved in spirits of wine, are also sold by the pint, all prepared, under the name of varnish, and in general are not costly. Those which the anatomist can turn from the ordinary arts to his own profits, are employed principally in pieces which it is intended to preserve dry. Perfect success attends the varnish, named in the shops fat, wood-red, à la copale, and with some others which remain a long time flexible.

These fluids are difficult to colour; it is necessary, for the first, to grind the colouring matter with essence, and for the others with alcohol; and to incorporate them afterwards with varnish after having slightly heated them. The carmined lakes, thus suspended in fat varnish, produce absolutely the effect of arterial blood; this colour preserves very well, and with such like injections it is unnecessary to colour the surface of the arteries.

The mixture of mutton fat or of suet, of white or yellow wax, of the fixed oils of olives, nuts, or flax seed, are the ordinary matters of injections, even for those destined for corrosions. The different degrees of solidity or softness are determined by the calculated proportions of wax and oil, and by the amalgam of resinous and colouring matters.

In general, in this sort of injections it is advantageous to introduce beforehand, a small quantity of volatile oil mixed with the fatty matter which is to serve for filling the vessels; by this preliminary process a liquid more fluid, more penetrating, higher coloured, and susceptible of cooling more slowly, is driven before into the smaller ramifications.

I could here transcribe several receipts proper to indicate the proportion of fatty matters among themselves; but the season during which the pieces are made, the nature of the ingredients employed on them, will occasion the proportional quantities to vary, so that a sketch only can be given for obtaining a matter which may be made more solid or more fluid after having tried it by cooling some drops separately. Nevertheless, here is one of those receipts:

℞.Of suet,5parts.
Burgundy pitch,2"
Oil of olives, or of nuts,2"
Of fluid turpentine and colouring matter, dissolved in volatile oil,1"

This latter part should not be mixed until the liquor is well melted and ready to be put into the syringe, as the heat will volatilize the volatile oils, which will become disengaged in the form of gas, and cause the mass to occupy a very great volume.