CRUSTS, ii chemistry. By crusts we understand those bony coverings,of which the whole external surface of crabs, lob sters, and other similar sea animals, are composed. Mr. Hatchett found them composed of three ingredients : 1. A car tilaginous substance possessing the pro perties of coagulated albumen. 2. Carbo nate of lime. 3. Phosphate of lime. By the presence of this last substance they are essentially distinguished from shells, and by the great excess of carbonate of lime above the phosphate they are equally distinguished from bones. Thus the crusts lie intermediate between bones and shells, partaking of the properties and constitu tion of each. The shells of the eggs of fowls must be referred likewise to the class of crusts, since they contain both phosphate and carbonate of lime. The animal cement in them, however, is much smaller in quantity. From experiments it is extremely probable that the shells of snails are composed likewise of the same ingredients, phosphate of lime hav ing been detected in them by these che mists.
Mr. Hatchett examined the crusts of crabs, lobsters, prawns, and crayfish. When immersed in diluted nitric acids, these crusts effervesced a little, and 5ra dually assumed the form of a yellowish white soft elastic cartilage, retaining the form of the crust. The solution yielded a precipitate to acetate of lead, and am monia threw down phosphate of lime. Carbonate of ammonia threw down a much more copious precipitate of carbo nate of lime.
On examining the crust which covers different species of echini, Mr. Hatchett found it to correspond with the other crusts in its composition. Some species of starfish yieldeth phosphate of lime, others none : hence the covering of that genus of animals seems to be intermedi ate between shell and crust.