26 



SCIENTIFIC NEWS. 



[July 13, 1888. 



In the case of the tilt-hammer, when applied for welding 

 a " faggot " of steel strips together, the success of the pro- 

 ceeding is dependent on the fact that the blows are dealt 

 in such rapid succession that the steel is maintained at a 

 welding heat, which it would lose if exposed for the same 

 length of time without the hammering. 



In these cases the hammer-head itself, although it is 

 the moving body whose motion is arrested, is but 

 slight!}' heated. The inner portion of its mass, its centre 

 of momentum, is practically unaffected, the only actual 

 effect on this being due to vibration communicated by 

 its elasticity and imperfect rigidity. The main heating 

 occurs within the mass of the yielding metal, where the 

 mechanical motion is arrested by internal friction. When 

 it becomes more rigid, and ceases to supply this internal 

 resistance, we have the well-known banging thuds that 

 shake the foundations of the solid earth. 



Die-sinking and coining afford another example. The 

 die is of hardened steel, and its descent is suddenly 

 arrested. Heat is thereby generated, but where ? Cer- 

 tainly not in the mass of the die. If so, the vigorous 

 proceedings at the Royal Mint would soon soften the hard 

 steel of which the dies are made. The coins are heated 

 considerably, and the metal of which they are composed 

 flows into the device on the face of the die. The 

 weight of metal thus heated in the course of a day's 

 work is many hundred times greater than that of the die 

 itself. 



When the blacksmiths of olden times lighted their 

 forge fires by converting the mechanical motion of their 

 hammers into heat-motion, it was not the hammer-head 

 that they applied to the brimstone match, but the piece 

 of soft iron that arrested the motion of the hammer. An 

 instructive experiment may be made by taking a leaden 

 bullet and a corresponding piece of iron, hammering each 

 on an anvil, and comparing the temperature of the vic- 

 tims. Both will be warmed, but the lead will be found to 

 become much hotter than the iron, and the hammer itself 

 not sensibly warmed. 



I contend that these facts, and a multitude of others 

 that might be specified, teach us that it is not the nega- 

 tive arrest of visible motion of the moving body, but the 

 positive conversion of this kind of motion into another 

 kind of motion, that effects the rise of temperature, and 

 that in all cases such conversion must and does occur 

 where the effective resistance occurs, whether that be on 

 the surface or within the substance or outside the mass 

 of the colliding bodies. (For further illustration of this, 

 see chapter 5 of my " Simple Treatise on Heat," published 

 by Chatto and Windus.) 



In the imaginary bodies of perfect rigidity it would be 

 all superficial, producing superficial fusion, volatilisation, 

 or dissociation according to quantity. In a perfectly 

 compressible and perfectly elastic body it would be all 

 internal, as is proved when we drive a close-fitting piston 

 into a closed cylinder of air or other gas, or, what amounts 

 to the same, if the piston be at rest and we drive the gas 

 in the cylinder towards it. Here the resistance occurs 

 internally, and the heat is therefore internal. 



If we apply a friction break to arrest the motion of a 

 solid fly-wheel, the heat appears on the surface where 

 the break is applied. If the fly-wheel be fluid, as a 

 vortex of gas or liquid, and the break be applied by a 

 resisting paddle within the fluid, the heat will appear 

 within the fluid, commencing at the place of impact with 

 the paddle, the heat being simply the other motion which 

 is there set up in exchange for the mechanical motion. 



THE "VOLTAIC BALANCE." 



ANEW and simple lecture experiment has recently 

 been devised by Dr. G. Gore, F.R.S. It is con- 

 ducted in the following manner : Take two small clean 

 glass cups containing distilled water ; simultaneously im- 

 merse in each a small voltaic couple, composed of either 

 unamalgamated magnesium or zinc, and platinum, taking 

 care that the two pieces of each metal are cut from the 

 same piece and are perfectly clean and alike. Oppose 

 the currents of the two couples to each other through a 

 sufficiently sensitive galvanometer, so that they balance 

 each other and the needle does not move. Now dip the 

 end of a slender glass rod in a very weak aqueous solu- 

 tion of chlorine, bromine, iodine, or hydrochloric acid, and 

 then into the water of one of the cups. The voltaic 

 balance is at once upset, as indicated by the movement 

 of the needle, and may be shown to a large audience 

 by means of the usual contrivances. 



The chief circumstance to be ndticed is the extremely 

 great degree of sensitiveness of the arrangement in cer- 

 tain cases ; this is shown by the following instances of 

 the minimum proportions of substance required to upset 

 the balance, with an ordinary astatic galvanometer of 

 too ohms resistance, and with a Thomson's reflecting 

 one of 3,040 ohms resistance. 



1. Zinc and Platinum with Iodine. — With the astatic 

 galvanometer between one part of iodine in 3,100,000 

 and 3,520,970 parts of water. 



2. Zinc and Platinum with Hydrochloric Acid. — With 

 the astatic galvanometer between 1 in 9,300,000 and 

 9,388,185 parts; and with the reflecting one, between 1 

 in 15,500,000 and 23,250,000 parts. 



3. Magnesium and Platinum with Bromine. — With the 

 astatic galvanometer between 1 in 310,000,000 and 

 344,444,444 parts. 



4. Zinc and Platinum with Chlorine. — With the asta- 

 tic galvanometer between 1 in 1,264,000,000 and 

 1,300,000,000 parts. 



5. Magnesium and Platinum with Chlorine. — With the 

 astatic galvanometer between 1 in 17,000,000,000 and 

 17,612,000,000 parts; and with the reflecting one, 

 between 1 in 27,062,000,000 and 32,291,000,000 parts of 

 water. 



Every different soluble substance requires a different 

 proportion, and with unlike substances the difference of 

 proportion is extremely great. With solutions of neu- 

 tral salts, the proportion of substance required to upset 

 the balance is large ; for instance, with chlorate of 

 potash, a zinc platinum couple, and the astatic galvano- 

 meter, it lay between 1 part in 221 and 258 parts of 

 water. 



The degree of sensitiveness of the balance is usually 

 greater, the greater the degree of chemical affinity the 

 dissolved substance has for the positive metal, and the 

 less it has for the negative one. 



By first bringing the balance with a magnesium plati- 

 num couple and the astatic galvanometer nearly to the 

 upsetting point, by adding 1 part of chlorine to 17,612 

 million parts of water, and then increasing the propor- 

 tion to 1 in 17,000 millions, the influence of the differ- 

 ence, or of 1 part in 500 millions, can be distinctly detected. 



The Senses of Insects. — Franz Ruland (Zeitschri/t 

 fur Wissenschaftliche Zoologie) considers that insects 

 possess in their antennae a series of different olfactory 

 organs, specifically perceptive of different odours. 



