Page:Popular Science Monthly Volume 21.djvu/796

778 would dilute the nitric acid, but, as the sulphuric acid has a greater affinity for water, it takes it up, leaving the other of its normal strength. To proceed to the details: Around a brick chimney is placed a wooden trough, containing large earthenware pitchers in which is the acid mixture, the trough being filled with ice. On a shelf above this trough are placed bottles holding glycerine, communicating each with a pitcher below, by means of a small rubber tube, so arranged as to permit the passage of the liquid in a fine stream. The contents of the pitchers are kept in a constant state of agitation by a stream of cold dry air forced through them; as the reaction between the nitric acid and the glycerine goes on, great heat is evolved, accompanied by nitrous fumes extremely unpleasant and unhealthy to inhale; these fumes are drawn into the chimney through an overhanging hood, by means of the draught created by a furnace-fire at the bottom. Great care is necessary that the temperature be not allowed to rise above 48° Fahr., as there would then be danger of the newly formed nitroglycerine taking fire and exploding; constant attention must be paid to this point, each pitcher being tried with a thermometer at short intervals. The proportion of materials is about two pounds of glycerine to twenty pounds of the acid, and at the expiration of the process the glycerine and nearly all the nitric acid have disappeared, forming nearly four pounds of their compound; the sulphuric acid, diluted as before mentioned, also remains. The nitro-glycerine is now partly in solution and partly suspended in the acid mixture; the contents of the pitchers are poured through a strainer into a vat of water, which is kept in agitation by a stream of compressed air: after all the pitchers are emptied, the air is shut off and the mixture comes to rest, when the nitro-glycerine settles at the bottom of the vat, and the acid water is then drawn off. It is next taken in small quantities at a time, and carefully washed a number of times, until all the acid is washed out, and only the pure nitro-glycerine remains; in this state it is thin, oily, creamy white, and opaque, but, on being placed in jars and allowed to stand, it soon becomes transparent. It is now ready for use as an explosive agent; it has a sweet, aromatic, pungent taste, and possesses the very peculiar property of causing an extremely violent headache when placed in a small quantity upon the tongue, or any other portion of the skin, particularly upon the wrist. It has long been employed by homœopathic practitioners as a remedy in certain kinds of headaches. In those who work much with it, the tendency to headache is generally overcome, though not always, It freezes at about 40° Fahr., becoming a white, half-crystallized mass, which must be melted by the application of water at a temperature of about 100° Fahr. If perfectly pure—that is, if the washing has been so complete as to remove all traces of the acid—it can be kept for an indefinite period of time; and, while many cases of spontaneous decomposition have occurred in impure specimens, there has never been known such an instance,