18. The part to be injected must now be immersed in warm water, not, however, above 100° Fahrenheit, and be left until the whole is thoroughly warmed. Whilst this is being done, the coloured size must be made ready by the pot being placed in the tin bath of warm water, which must be of sufficient temperature (about 110° Fahrenheit) to keep it perfectly liquid. For the same purpose, the syringe is often tightly covered with two or three folds of flannel; and, indeed, there is no part of the process which requires more attention. If the substance to be injected is too hot, it is injured; whilst, if any of the articles are too cold, the gelatine, or size, loses a part of its fluidity, and consequently cannot enter the minute parts. When all is prepared, the syringe, with the stopcock attached, should be warmed, and then filled and emptied with the injecting fluid two or three times, care being taken that the end of the syringe be kept beneath any bubbles which form upon the surface. The syringe may then be filled, and closely attached to the pipe which is tied in the vessel. With a firm and steady pressure the piston must be forced downwards, when the substance will be perceived to swell, and the colour show itself in places where the covering is thin. When the syringe is _almost_ emptied of its contents, the stopcock must be turned to prevent any escape of the injection from the subject. It must then be refilled, as in the first instance, and the process repeated. I say _almost emptied_, because it is well not to force the piston of the syringe quite to the bottom, lest the small quantity of air which frequently remains be driven into some of the vessels, and the object be injured or quite ruined. As the injection is proceeded with, it will be found that the force required grows greater, yet care must be taken not to use too much, or the vessels will burst, and render all the labour fruitless. The movement of the piston must be occasionally so slow as to be almost imperceptible, and for this reason it is sometimes marked with lines about one-eighth of an inch apart.