82 



SCIENTIFIC NEWS. 



[June 1st, iS 



In bedrooms — if of such a size and shape that the current 

 of air does not blow directly over the person of the sleeper 

 — the windows should be left open all night, save in time 

 of fog, frost, etc. The notion that night air is essentially 

 unwholesome holds good only in districts where malaria 

 reigns. 



(To be continued.) 



THE CONDUCTIVITY OF METALS. 



WE are all more or less familiar with the fact that different 

 substances vary greatly in their powers of conducting 

 heit. Organic substances such as wood, cotton, wool, etc., 

 are bad conductors, and it is chiefly for this reason that 

 nature has clothed birds and animals with feathers and fur, 

 as they help to keep up the necessary temperature of the 

 body. Metals on the other hand are good conductors of heat, 

 as we know by varied experience ; if we wish to lift a hot 

 smoothing iron we take good care to cover the handle first 

 with a piece of flannel or other bad conductor, and for the 

 same reason the handles of metal tea-pots are made of wood 

 or ivory. Many other instances could be given, but it 



Fig. I. — Muslin over metal ball not burnt by red-hot cinder 

 on top. 



is needless, as they will readily occur to any one whose 

 attention is directed to the point. Recently La Nature has 

 given two rather striking examples, which are shown in the 

 accompanying illustrations. If a ball of copper or other 

 metal some three or four inches in diameter be covered 

 tightly with thin muslin or cambric, as shown in Fig. i, a 

 red-hot cinder from the fire can be placed on the top of the 

 ball, and it can be made to glow by breathing on it, without the 

 muslin or cambric cover being injured. This is simply due 

 to the fact that the metal ball conducts heat away so rapidly 

 from the part next to the hot cinder that the cotton covering, 

 although very inflammable, never becomes hot enough to 

 burn. In Fig. 2 we have shown a piece of muslin or 

 cambric stretched tightly over the end of a gas burner : if 

 the burner is metal, and if the cotton covering is without 



creases at the top, the gas can be turned on and lighted 

 without the covering being burnt. In this case also the ex- 

 planation is a very simple one : the metal burner conducts 

 away the heat of the flame so rapidly that the cotton cover- 

 ing never reaches a temperature high enough to injure it. 



Fig. 2. — Muslin over end of gas-burner not injured by flame. 



OSMOSE. 



IT has been known for some time that when two liquids 

 are separated by a thin porous partition, such as a sheet 

 of animal membrane, unglazed earthenware, or porous stone, 

 the liquids gradually pass from one side to the other of the 

 partition. If, for instance, a solution of common salt be 

 on the one side of the division, and water only on the 

 other, the salt will pass in one direction and the water in 

 the other. Or if a membranous bag is filled with a strong 

 syrup, or some other fluid denser than water, such as 

 milk or albumen, and if the bag thus filled is placed in a 

 vessel containing water, the following change will take 

 place : — Water will gradually find its way into the bag, 

 and by degrees the water on the outside of the bag will 

 contain some of the syrup or other fluid put into the 

 bag. If, on the other hand, the bag is filled with water, 

 and immersed in a syrup or other fluid denser than 

 water, gradually water will leave the bag, and syrup will 

 pass into it. The passage of liquids to the two opposite 

 sides of the partition will not take place at the same rate, 

 and the flow of the liquid towards that which increases in 

 volume is sometimes called cndosmose, and the flow in the 

 opposite direction cxosmosc, but for general purposes it is 

 sufficient to adopt the single word osmose (from wcr/tios, 

 impulsion) to denote the interchange of the fluids. Osmose 

 plays a most important part in organic life, the cell-walls 

 being the diaphragms through which the necessary inter- 

 change of fluids and dissolved substances takes place, as in 

 the passage of the products of digestion from the digestive 

 cavities into the vessels containing the circulating materials. 

 Many manufacturing processes depend on these phe- 

 nomena, but for the moment we will merely describe a use- 



