The best means of preventing infection, are ventilation and cleanliness in every particular. The best means of destroying it are those powerful chemical agents which have the power of uniting with the hydrogen which is supposed to form part of the infectious substances. Nitric acid gas, formed by pouring oil of vitriol on nitre or saltpetre, has been used; but though efficacious, it possesses several disadvantages, being irritating to the lungs, corrosive to metal-work, and also, when largely employed, very expensive. The most powerful, easily controlled, and in every sense the best disinfectant, is chlorine gas. This agent at once destroys every trace of infection in all substances submitted to its action. Its formation is perfectly under control, and goes on in a gentle manner for days together, without requiring care or attendance. We consider that the slow liberation of chlorine is far superior to the employment of chloride of lime, which gives forth the gas in a modified form. In our own experience, we have employed it to destroy various infections, and always with complete success. In one case of a school where scarlet fever had returned after several attempts at purification, and in the last instance with a fatal effect, we used chlorine, and effected the complete removal of every trace of the disease. Various modes of liberating chlorine are known to chemists; but, for such purposes as the present, where a slow, uniform, and constant action is required, there are none equal to the following plan:—One pound of common table-salt is to be intimately mixed by stirring with an equal weight of a substance called manganese, which may be readily obtained from any good chemist. Small portions of this mixture should be placed in shallow pans (the saucers of common flower-pots answer the purpose exceedingly well); and upon them should be poured a mixture of oil of vitriol (sulphuric acid) and water, the quantity required for the above weights, viz., for one pound of each ingredient, being two pounds of oil of vitriol and one of water, both by weight. These should have been previously mixed in a wooden vessel, being stirred by a wooden lath, and allowed to become cool before being poured on the salt and manganese, as the mixing of the acid and water generates great heat. Too much care cannot be taken with the acid, as it is excessively corrosive in its nature, and destroys most substances with which it comes in contact. When these materials are all mixed, chlorine is slowly evolved for a period of three or four days, and in so gentle a manner, that not the slightest irritating or unpleasant effect is produced. If it is wished to cause the more rapid production of gas, the saucer may be placed over a basin of boiling water, or upon a hot brick; but the slow generation for a considerable length of time is what should be more especially aimed at. It is needless to say, that all substances supposed to have been contaminated, should be spread out