i8o 



SCIENTIFIC NE^VS. 



[Oct. 1st, iS 



Liebig asked himself the question, Does every kind of food 

 go to the production of heat ; or can we distinguish, on the 

 one hand, between the kind of food which goes to create 

 warmth, and, on the other, that by the oxidation of which 

 the motions and mechanical energy of the body are kept 

 up ? He thought he was able to do this, and he divided 

 food into two categories ; the starchy and fatty food 

 is that, said he, which by its combustion provides the 

 warmth necessary for the existence and life of the body. 

 The albuminous or nitrogenous constituents of our food, 

 the flesh meat, the gluten, the casein out of which our 

 muscles are built up, are not available for the purposes of 

 creating warmth, but it is by the waste of those innsdes that 

 the mechanical energy, the activity, the motions of the 

 animal are supplied. We see, said Liebig, that the Esqui- 

 maux feeds on fat and tallow, and this burning in his body 

 keeps out the cold. The Gaucho, riding on the pampas, 

 lives entirely on dried meat, and the rowing man and pugi- 

 list, trained on beef steaks and porter, require little food to 

 keep up the temperature of their bodies, but much to 

 enable them to meet the demand for fresh muscular tissue, 

 and for this purpose they need to live on a strongly nitro- 

 genous diet. 



This was the teaching of so able a chemist and physio- 

 logist as Liebig, but modern science has proved that his 

 hypothesis was a wrong one, and we now have good reason for 

 supposing that no such distinction as he laid down can hold 

 good. Muscle is a nitrogenous substance, and if it were 

 oxidised as Liebig supposed, it would yield a' nitrogenous 

 excretion representing the oxidised nitrogenous substance. 

 All the nitrogen which is excreted from the body is re- 

 cognised chemically as urea, uric acid, etc., and if Liebig 

 were correct, the work done by the body should be repre- 

 sented by the amount of these nitrogenous compounds 

 excreted. This, however, is not the case, for the urea, etc., 

 is not increased in amount in proportion to the work, as 

 was proved by the well-known Faulhorn experiments of 

 Messrs. Kick and Wiscelenus. They, in fact, proved that 

 the actual energy represented by the mechanical work of 

 raising the body to the summit of this mountain was twice 

 as great as that which could possibly be produced by the 

 oxidation of the nitrogenous constituents eliminated from 

 the body during twenty-four hours. That is to say, 

 taking the amount of nitrogenous substance cast off from 

 the body, not only whilst the work was being done but 

 during twenty-four hours, the mechanical effect capable of 

 being produced by the combustion of muscular tissue, from 

 which this cast-off material was derived, would only have 

 raised the body half way up the Faulhorn. 



On the other hand the excretion of carbonic acid and water 

 has been proved to be in proportion to the amount of muscular 

 exertion, and as the heat value of the carbon and hydrogen 

 it represents is easily determined, and Dr. Joule has taught 

 us how to obtain the mechanical equivalent of heat, we are 

 able to measure the work done by the oxidation of these con- 

 stituents of the body in foot pounds. In a series of experi- 

 ments made by Dr. Frankland, he was able to determine the 

 quantity of energy or force manifested as heat during the 

 oxidation of a given weight of alimentary substance, and in 

 this way he was able to estimate the comparative value of 

 different food substances equivalent to the muscular force 

 which would raise a man of ten-stone weight 10,000 feet, 

 this being the estimated daily expenditure of force in work- 

 ing the machinery of the body. 



Reverting to Liebig's hypothesis, we can now see that it 

 is not substantiated, and that although the nitrogenous con- 

 stituents of the food do doubtless go to repair the waste of 

 muscle, which, like every other living tissue, wears out and 

 needs renewal, the non-nitrogenous food not only supplies 



animal heat, but also furnishes by its oxidation the muscular 

 energy of the body. The solid h3-drocarbons, starch and 

 oil, after digestion and incorporation with the tissues, on 

 being converted into carbonic acid and water, supply the 

 heat energy of the body, just as the carbon and hydro- 

 carbon in coal supplies heat energy to a steam engine. The 

 nitrogenous constituents, however, are used, as we observed 

 above, to make good the wear and tear of the nitrogenous 

 tissues ; and it must be remembered that the muscles wear 

 out faster when doing work, and that the amount of nitro- 

 genous food must therefore bear due proportion to the 

 amount of work performed. 



THE TELEPHONE: ITS PRINCIPLES, 

 CONSTRUCTION, & APPLICATION. -IIL 



"X^rE must return to the subject of variable resistance 

 » »' transmitters, and see how the principle involved is 

 practically applied at present. Edison brought out a trans- 

 mitter in which the contact of variable resistance is between 

 a platinum plate and a little button of compressed lamp- 

 black. It is illustrated in Fig. 9.* 



Fig. 9. Edison's Transmitter. 



The case is ot ebonite, containing the diaphragm D, 

 carrying on it a cork pad attached to the platinum disc P, 

 between which and a similar disc P' is the little button of 

 carbon, C. The screw, S, adjusts the pressure on the con- 

 tacts between C and PP', and the elasticity of the cork and 

 the carbon button are sufficient to prevent any break in the 

 contact, and yet to allow the variations of pressure to be 

 transmitted to the contacts. Part of the effect of this trans- 

 mitter is probably due to a lessening of the resistance of the 

 carbon button itself upon compression, and Edison himself 

 claims this as the principal action. This theory is, how- 

 ever, hardly borne out by later experiments. 



Edison suggested the use of a button of silk or similar 

 elastic fibre impregnated with powdered graphite (black- 

 lead), but no instrument with such a material is in practical 

 use, and there are comparatively few Edison transmitters 

 now employed. This is not because the Edison transmitter 

 is not a good one, but because the soft carbon button is 

 very fragile, and the hard carbon transmitters about to be 

 described are more easily kept in adjustment and repair. 



The two forms of carbon transmitter chiefly employed in 

 the United Kingdom are the Gower-Bell and the Blake in- 

 struments. The former is used by the Postal Telegraph 

 department. The latter is the instrument adopted by the 

 United Telephone Company and its offspring. 



The Gower-Bell is what is known as a microphone 



* We are indebted to Messrs. Longmans, Green and Co. for the 

 illustrations of Figs. 9, 10, and II, and to the Secretary of the Society 

 of Telegraph Engineers and Electricians for Fig. 12. 



