Page:Popular Science Monthly Volume 47.djvu/731

Rh roof of a cave and dripping slowly to the floor beneath. In cases where the water filters sufficiently slowly or evaporation is correspondingly rapid, the deposit of lime carbonate from the roof takes at first the form of a ring around the outer portion of the drop, a natural consequence of the evaporation of a suspended drop of liquid. This process may go on until the ring becomes prolonged into an elongated cylinder or tube, the diameter of which may not exceed five millimetres, though usually ranging from five to ten, and of all lengths up to fifty centimetres. In exceptional cases this length may be exceeded, but owing to the delicacy of the material the stalactite usually breaks from its own weight and falls to the floor before the length of-even ten or fifteen centimetres is reached, to become imbedded in the stalagmitic material there forming. Lengths of even these dimensions are comparatively rare, for the reason that the tube becomes shortly closed, either as its upper or lower end, usually the upper, and all growth from the extremity alone ceases, subsequent depositions being wholly exterior and taking place in the form of concentric coatings of the carbonate on the outer surface and at the same time from the top. There is thus formed around the original tube a compact cylindrical mass, in its typical form, constricted at the point of attachment, but thickening rapidly and then tapering gradually into an elongated cone. The material of the stalactites is not always wholly carbonate of lime, but in some cases thin intervening coats of iron disulphide are met with. Through a kind of crystallization the material sometimes undergoes a distinctly fibrous arrangement, but oftentimes the structure is granular throughout.

Snake-bite Antitoxine.—At a recent meeting of the Royal Society of Edinburgh Prof. Fraser delivered a lecture embodying some extremely valuable and interesting data obtained by him during several years of experimental work on an antidote for snake poisons. The principles utilized by him are similar to those employed in the antitoxine treatment of diphtheria and in vaccination for smallpox. He first immunized an animal by repeated small doses of the snake poison, slowly increasing the quantity, until the animal was taking at a single dose many times the minimum lethal amount for a non-immunized individual. He then injected into another animal some of the blood serum from the immunized case, and found that this prevented any ill effects from a subsequent injection of venom. Still a third animal was given an injection of pure venom, and, when distinct symptoms of poisoning appeared, was treated with the immunizing serum, with the result that the symptoms of poisoning disappeared and no ill effects followed. When it is remembered that in British India alone there are each year from eighteen to twenty thousand deaths caused by snake-bite, the great beneficence of this discovery is apparent. Prof. Fraser is at present immunizing a horse, but is having some trouble, owing to the difficulty of procuring the snake-poison in sufficient quantity.

Unsanitary Filters.—For many years before any positive connection was established between typhoid fever and a specific microorganism it was known that this and other diseases were in some way connected with the composition of the drinking water previously consumed by the patient. By chemical analysis it was found that in almost all such cases the water contained an excess of organic matter; it was accordingly inferred that removing the organic matter would correct the trouble and obviate any further danger; and filters were made with this end in view. It is now known, however, that the danger from waters containing much organic matter lies not in the organic matter per se, but arises from the fact that a large amount of organic matter attracts and feeds a proportionately large number of bacteria. It has been proved experimentally that after a filter of this class has been in use for some time, water, in passing through it, becomes much richer in bacteria, and even that sterilized water passed through it is found swarming with micro-organisms. The filter collects the organic matter from the water and with it some of the bacteria. This mass of organic matter serves as an admirable culture medium; as the bacteria multiply, they are taken up by the water as it passes through the filter, so that, instead of serving as a safeguard against disease, such filters are really disease breeders. In order to be