_An imperial gallon of water weighs_ 10 _pounds avoirdupois weight_. Water for dietetical purposes is obtained principally from three sources:—1. Rivers; 2. Surface wells; 3. Deep or Artesian wells. Water from all three sources contains saline or mineral matters in solution, and, provided they are not in quantities so large as to act injuriously on the system, water may become a source of supply of these constituents to the body. The best remedy for impure water is filtering, which may be done by passing the water through charcoal and sand. “A Poor Man’s Filter” is exhibited in the Museum, which can be very easily and cheaply constructed by using a common flower-pot, glazed inside, plugging the drainage hole (not too tightly) with a piece of clean sponge, then adding layers of animal charcoal, sand, and rather coarse gravel. Filters from the establishments of the Messrs. Lipscombe, the Messrs. Ransome, and the Carbon Filter Company are also exhibited. The passing water over iron has been found to have a remarkably purifying effect, and this has been patented by Dr. Medlock. The _organic_ impurities of water are best tested by the aid of the microscope, but, as an examination by this instrument requires much time, a ready method of obtaining a knowledge of the comparative organic impurity of waters is the addition of the permanganate of soda or potash. This salt, which gives to water a beautiful red colour, is easily decomposed by organic matters. When the same quantity of the permanganate is added to a series of waters containing organic matters, those which contain the least retain the most colour and _vice versâ_. Waters thus tested are exhibited in the Collection. Water from the chalk or limestone is generally hard, arising from its holding in solution carbonate of lime, which, although insoluble in water, is dissolved by the agency of carbonic acid. By Clark’s softening process the carbonic acid is neutralized by lime, and the carbonate of lime is thus thrown down. Specimens are exhibited. Water is frequently stored in leaden cisterns, and when free from carbonic and phosphoric acids it acts powerfully on lead. Thus distilled water becomes speedily tainted with lead, whilst Thames water and London surface well water act but slightly upon it. Specimens of these waters acting upon lead are exhibited in the Collection. SALT. Common Salt is a chloride of sodium, and exerts an extraordinary influence on animal as well as vegetable life. All marine animals and plants seem to have their existence determined by this substance. It enters into the composition of the human body, and all over the world man uses it, when he can obtain it, in its mineral form, as an addition to his food. In Case 5 will be seen a collection of salt from various countries. MINERALS IN FOOD.