The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Analytical Portraiture
precipitate on adding ammonia, add a little chloride of ammonium and carbonate of ammonia. A white precipitate indicates calcium, or a metal (strontium, barium) of the calcium group. The acid of a salt may often be recognised by putting a few grains of the salt in a test tube and adding a little sulohuric acid. Effervescence, with odourless gas, indicates a carbonate ; with smell of burning sulphur, a sulphite or hyposulphite, white flocks of sulphur being liberated in the fluid in this latter case. Smell of hydrocyanic acid (which to some recalls bitter-almond oil) indicates a cyanide. If no change, apply heat. Fumes of hydrochloric acid, recognisable by small white clouds, if rod moistened with ammonia is introduced, and producing tur-bidity in drop of silver nitrate on glass rod, indicate a chloride. Bright yellow coloration, with liberation of a yellow gas which explodes when heated, indicates a chlorate. Violet fumes indi-cate an iodide ; odour of vinegar suggests an acetate.
Too much confidence must not be placed in the results obtained by such an examination as is outlined above, as the conditions under which each reaction takes place require to be minutely studied ; but such an examination may occasionally be useful in assisting or confirming the memory with respect to unlabelled materials. The so-called test tubes of thin glass are used in trying the reactions, and at every stage care and caution is needed. The face should never be held over the mouth of the test tube, fumes being smelled by diverting a little with the hand. The mention of an explosive gas in connection with chlorates suggests the precautionary measure of always using very small quantities in the first trial of a reaction. (As bearing on this subject, see Hydrometry and Hydrometers ; also Equivalence, Chemical.)
Analytical Portraiture. Starting with the fact that a properly adjusted negative and positive, if superimposed, will neutralise each other, Mr. Galton deducts the normal face from the "smiling" face or the ''glum" face, thus obtaining differ-ences, or, as he terms them respectively, the " transformer of a smile," or the M transformer of glumness." (For further details see The Amateur Photographer, August ioth, 1900, p. 106 ; and for an account of Mr. Galton's experiments in a collateral direction, see Composite Portraiture.)