706 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



the pasteboard cover of the 13th experiment. N° 3, the thermometer placed in its focus, a, the prism. 

 c, semicircular cover, used in the 17th and 18th experiments, instead of the one with a square 

 hole. 



Fig. 8, a, the burning lens of the 14th experiment, b, the fire in the chimney. N° I, the ther- 

 mometer in the focus of the lens. N° 4, the standard thermometer, c, the hole through which the 

 rays of heat pass to N° 1. d and e, two holes, between which the ball of the thermometer N° 4 is 

 screened from the direct rays of the fire ; while free access is given to the heat which may affect the 

 temperature of the place. 



Fig. 9, a, the iron cylinder, stuck on its handle, as it is used in the 15th and 19th experiments, b, the 

 lens, c, the screen with an opening in it. N° 2, the thermometer in the focus of the lens. N°3, the 

 standard thermometer, d, the little moveable pasteboard screen. 



Fig. 10, a b, the front, plated with iron, that it may bear to be exposed close to the bars of a chimney 

 fire, c, the concave mirror, d, the plain mirror, e, the lens. N° 1, the thermometer in the focus 

 of the lens. N° 4, the standard thermometer, f, a circular opening in the front plate a b, for ad- 

 mitting the rays of heat to fall on the concave mirror c. m, the first focus of the rays, from which 

 they go on diverging, to the small mirror, and to the lens ; which brings them to a 2d focus, on the ball 

 of the thermometer N° 1. 



XVI. Chemical Experiments on Zoophytes ; with some Observations on the Com- 

 ponent Parts of Membrane. By Chas. Hatchett, Esq. F. R. S. p. 327. 



The experiments and observations on shell and bone, which I last year laid be- 

 fore the r. s., were made in consequence of my having a little before discovered 

 that the enamel of teeth did not consist principally of carbonate of lime, but was 

 of a nature similar to bone ; with this difference, that the phosphate of lime was 

 not deposited in and upon a cartilaginous or membranaceous substance, but was 

 only blended with a certain portion of animal gluten. By the experiments subse- 

 quently made on various shells, crustaceous substances, and bones, it was proved, 

 1st. That the porcellaneous shells resemble the enamel of teeth in the mode of 

 formation, but that the hardening substance is carbonate of lime. 2dly. That 

 shells composed of nacre or mother of pearl, or approaching to the nature of that 

 substance, and also pearls, resemble bone in a considerable degree, as they consist 

 of a gelatinous, cartilaginous, or membranaceous substance, forming a series of 

 gradations, from a tender and scarcely perceptible jelly to membranes completely 

 organized, in and upon which carbonate of lime is secreted and deposited, after 

 the manner that phosphate of lime is in the bones; and therefore, as the porcella- 

 neous shells resemble the enamel of teeth, so the shells formed of mother of pearl, 

 &c. in like manner resemble bone; the distinguishing chemical character of the 

 shells being carbonate of lime, and that of enamel and bones being phosphate of 

 lime. 3dly. It was proved, that the crust which covers certain marine animals, 

 such as crabs, lobsters, crayfish, and prawns, consists of a strong cartilage, hardened 

 by a mixture of carbonate and phosphate of lime; and that thus these crustaceous 

 bodies occupy a middle place between shell and bone, though they incline prin- 

 cipally to the nature of shell. And, 4thly. That a certain portion of carbonate of 

 lime enters the composition of bones in general ; the proportion of it however being, 

 to the phosphate of lime, vice versa to that observed in the crustaceous marine 



