CORRODED PREPARATIONS. 389 



the great concern is the perfectness of the cast, and the goodness of its 

 consistence for such purpose. 



The substances for this purpose might be as various as any other in 

 the art of casting, if our moulds admit of it. We might have casts of 

 all the metals or any other substance that admits of fusion or fluidity 

 in any way, and afterwards becomes solid ; but such is the nature of 

 most of the animal moulds, that they cannot support a heat sufficient to 

 allow of carrying this art to any great length. 



The parts to be injected are the first consideration. They must be 

 extremely sound or free of all disease, and they should be very fresh 

 or free from putrefaction. The parts to be injected should be taken 

 out of the body with great care, so as to have the principal trunk or 

 trunks of the clifferent systems of vessels preserved for the fixing the 

 pipes ; for few parts that are injected for corrosion anastomose so freely 

 as to admit such injections to pass from one branch to a neighbouring 

 one ; therefore there will be no more injected than the part to which 

 this vessel went or supplied. Besides preserving the principal trunks, 

 it gives distinctness to the different systems of vessels, and an elegance 

 to the preparation. The different systems of vessels or cavities are to 

 be considered, by way of a leading step to the different coloured in- 

 jections. 



The injection is the next consideration, and as it is wholly a piece of 

 art, and the only thing which is to be saved, it becomes an object to 

 carry it to the greatest perfection we can. The first property of an 

 injection is fluidity at one time under certain circumstances, and solidity 

 at another. The nearer a substance can be brought to these two states, 

 the more it is calculated for an injection in this respect ; but there are 

 many substances which have these two properties to a great degree, yet 

 the circumstances necessary to bring them to such a state may make 

 them very unfit. 



Every injection for this purpose owes its fluidity to heat, and its 

 solidity to cold (excepting plaster of paris), therefore the degree of 

 fluidity necessary, and the degree of heat the parts will admit of, are 

 the things to be considered. The degree of solidity when cold is to be 

 regulated according to the degrees of heat of the atmosphere in which 

 the preparation is made or to remain in ; for there are many substances, 

 such as many of the metals, whose heat when fluid is by much too great 

 for an animal substance, although very proper when cold ; while there 

 is another metal, viz. mercury, whose heat in a fluid state is very proper, 

 but whose solid state is not to be procured by the heat of the atmo- 

 sphere in which the preparation is made or to remain, nor indeed by 

 any cold we are capable of producing. The degree of heat of an injec- 



