CRYSTALLIZATION. POLARIZATION. 325 



liquid, but at diametrically opposite points, the solid salts intended to 

 act on one another by double decomposition, such Muriate, Nitrate, or 

 Acetate of Lime, and Carbonate of Potass or Soda; so that, being very 

 gradually dissolved, the two substances may come slowly to act upon 

 each other, and may throw down their precipitate in the midst of the 

 ' colloid.' The whole is then covered with a plate of glass, and left for 

 some days in a state of perfect tranquillity; when there begin to appear 

 at various spots on the surface, minute points reflecting light, which 

 gradually increase and coalesce, so as to form a crust that comes to adhere 

 to the border of the plate; whilst another portion of the precipitate sub- 

 sides, and covers the bottom of the plate. Round the two spots where 

 the salts are placed in the first instance, the calcareous deposits have a 

 different character; so that in the same experiment several very dis- 

 tinct products are generally obtained, each in some particular spot. The 

 length of time requisite is found to vary with the temperature, being gen- 

 erally from two to eight weeks. By the introduction of such a coloring 

 matter as madder, log-wood, or carmine, the concretions take the hue 

 of the one employed. When these concretions are treated with dilute 

 acid, so that their calcareous particles are wholly dissolved-out, there is 

 found to remain a basis-substance which preserves the form of each; 

 this, which consists of the * colloid' somewhat modified, is termed by 

 Harting calco-globuline. Besides the globular concretions with the pecu- 

 liar concentric and radiating arrangement obtained by Mr. Rainey (Fig. 

 499), Prof. Harting obtained a great variety of forms bearing a more or 

 less close resemblance to the following: 1. The 'discoliths' and ' cya- 

 tholiths' of Prof. Huxley (Fig. 293). 2. The tuberculated 'spicules' 

 of Alcyonaria (Figs. 362, 363), and the very similar spicules in the man- 

 tle of some species of Doris ( 573). Lamellae of ' prismatic shell-sub- 

 stance' ( 363), which are very closely imitated by crusts formed of 

 flattened polyhedra, found on the surface of the ' colloid.' 4. The 

 spheroidal concretions which form a sort of rudimentary shell within 

 the body of Limax ( 573). 5. The sinuous lamellae which intervene 

 between the parallel plates of the ' sepiostaire ' of the Cuttle-fish ( 575); 

 the imitation of this being singularly exact. 6. The calcareous concre- 

 tions that give solidity to the ' shell ' of the bird's egg: the semblance of 

 which Prof. Harting was able to produce in situ, by dissolving away the 

 calcareous component of the egg-shell by dilute acid, then immersing 

 the entire egg in a concentrated solution of chloride of calcium, and trans- 

 ferring it thence to a concentrated solution of carbonate of potass, with 

 which, in some cases, a little phosphate of soda was mixed. 1 Other forms 

 of remarkable regularity and definiteness, differing entirely from anything 

 that ordinary crystallization would produce, but not known to have their 

 parallels in living bodies, have been obtained by Prof. Harting. Look- 

 ing to the relations between the calcareous deposits in the scales of Fishes 

 (| 657-659) and those by which Bones and Teeth are solidified, it can 

 scarcely be doubted that the principle of * molecular coalescence ' is appli- 

 cable to the latter, as well as to the former; and that an extension and 

 variation of this method of experimenting would throw much light on 

 the process of ossification and tooth-formation. The inquiry has been 



1 See Prof. Harting's "Recherches de Morphologic Synthetique sur la produc- 

 tion artificielle de quelques Formations Calcaires Inorganiques, publiees par 

 1' Academic Royale Nederlandaise des Sciences," Amsterdam, 1872; and <; Quart. 

 Journ. of Microsc. Sci.," Vol. xii., p. 118. 



