The two principal modifications of gelatine are those produced from bones, sinews and skins of animals, and from the air bladder of the sturgeon, the latter variety, better known under the name of isinglass, represents, from a chemical point of view, glue in the purest form. On account of its high price and ever-varying quality as found in commerce, it must in practice give place to a glue or size containing chondrin, and prepared from bone and gristle. Another objection is, that plates prepared with an admixture of isinglass are less durable than those containing gelatine only. On the other hand, it certainly possesses advantages of its own, which will be more fully considered hereafter. A gelatine suitable for collotype must absorb plenty of water and swell out, but not liquefy after having been soaked in cold water for twenty-four hours. Gelatine or isinglass prepared from the bladder will not stand this test, but, on the contrary, partially disintegrate. The consistency and bearing capacity of a jelly formed by dissolving gelatine in water by means of heat and allowing it to cool, form, in the opinion of Lipowitz, a safe and reliable means of forming a judgment as to its suitability for collotype. Dissolve by first soaking five grammes in 45 cm. of distilled water for several hours, afterwards apply heat, and allow to stand twelve hours to set. For purposes of comparison this should always be done in a glass of the same diameter. A small tin or zinc slip is placed across the top of the glass vessel containing the jelly; through the centre of the tin a hole is pierced, through which a wire moves easily up and down. To its bottom end is soldered a piece of tin, of about 15 cm. diameter, the convex side lying upon the surface of the gelatine. On the other or upper end a small funnel is placed, large enough to contain 1-1/2 kilo. of small shot. The weight of the wire and funnel being ascertained, it will be a simple matter, by also weighing the shot, to calculate the weight required to sink the instrument into the jelly, and to ascertain with accuracy its weight-bearing capacity and proof of the relative value of various samples for Collotype printing. If otherwise suitable, the variety capable of supporting the greatest weight before yielding will be the one to select. The power of absorption of different samples varies considerably; for example, the Collotype gelatine of Creutz absorbs in two hours 30 parts of cold water poured over it. Nelson’s patent opaque, in the meantime, only absorbs five parts, but after a further period of 4-1/2 hours the difference is equalised, as it will absorb 25 parts more of water, making altogether exactly 30 parts. This experiment proves the latter to be a harder quality than that of Creutz, and it necessarily requires a somewhat different treatment for the production of the best results.