CHLOR0-10DO-BROMIDE EMULSION.—An emulsion containing a chloride, an iodide, and a bromide.
CHLOROPHYL.—The peculiar substance to which the green color of the leaves, fruit and other parts of plants is due, and the nature of which, notwithstanding the extent of the researches that have been made, is still doubtful. It dissolves in alcohol and ether, but not in water. Its properties are neutral, being neither of an acid nor a base.
Pelletier was the first to give the substance its name, and thought it to be a peculiar proxi mate principle; later on he found it to be a mixture of wax and a green colored oil. Mons. Fremy succeeded, by the joint action of two parts of ether and one of hydrochloric acid, diluted with water, to separate chlorophyl into two distinct coloring principles, which he called phylltan thin, because of its yellow color, and the other phyllocyanine because of its blue. If these two are separated from their solution and reunited, the result is a green color, the chlorophyl of the first instance.
Under the continuous action of light and heat upon vegetables, chlorophyl splits into these constituents, by which the change of color in autumnal leaves is easily explained. That not all leaves turn to a bright yellow or orange, like those of the maple tree, has its reason in the fact of the presence of certain chemical reagents. Tannin in the oak leaves for example makes the color a yellowish brown, the leaves of the willow turn almost white because of the salicine present Rhus, clematis, and other plants turn from bright red to a lively purple for similar reasons.
Becquerel was the first to use chlorophyl for photographic purposes, as a red sensitizer. Ives followed in 187%* and perhaps employed it far more extensively than any other photographer ever did. But chlorophyl gave satisfactory results mainly in the wet collodion process; with gelatine emulsion plates it was far less useful.
Experimenters have stated that chlorophyl forms an intimate combination with caustic alkalis, and is, in that state, a fine sensitizer for red.
Chlorophyl has, however, been superseded by more convenient reagents, like chinoline blue, or malachite green, and at present little is heard of it.
Books on chemistry, and our own experience, have proved that the best source to obtain the substance is spinach leaves. Ives made it from " vinca minor," called by a provincial name, garden myrtle.
To make chlorophyl, fresh and juicy leaves of plants are pounded in a mortar, and, when reduced to a pulpy state, the liquid is separated from the solids by filtration or straining. When digesting this liquid with ether, chlorophyl and waxy substances are dissolved. They are evaporated to dryness, and by a treatment with alcohol the waxy substances will separate. The solution, after being filtered, is again evaporated to dryness, and dissolved in strong, hydrochloric acid, which gives a solution of beautiful , reen color with a very lively red fluorescence.
Marble dust, carbonate of calcium, throws from this solution a flocculent green precipitate, not soluble in water.
The chemical constitution of chlorophyl has not yet been fully determined upon, but it contains much nitrogen.
Chlorophyl has also been found in the animal kingdom, in animals of the lowest order.
Solutions of chlorophyl show a bright green color and an absorption spectrum that is exceedingly interesting and characteristic. It absorbs nearly the whole spectrum, only a narrow strip of light at the extreme red being visible. This peculiar property has made it of great use in orthochromatic photography in rendering silver bromide more sensitive to the red end of the spectrum. Ives' process is entirely based upon the uses of this substance. See Orthochromatic Photography.