114 ANNUAL OF SCIENTIFIC DISCOVERY. 



preparation for commerce, and the difference of specimens depends 

 probably upon the manner in which these are applied, and followed by 

 the after-process of rolling between hot cylinders. However if a 

 portion of that which conducts be warmed in a current of warm air, 

 as over the glass of a low gas-flame, and be stretched, doubled up, 

 and kneaded for some time between the tingers, as if with the inten- 

 tion of dissipating the moisture within, it becomes as good an insu- 

 lator as the best. 



Dr. Faraday soaked a good piece in water for an hour, and on tak- 

 ing it out, wiping it, and exposing it to the air for a minute or two, 

 found it insulated as well as ever. Another piece was soaked for four 

 days, and then wiped and tried : at first it was found lowered in insu- 

 lating power, but after twelve hours' exposure to the air, under com- 

 mon circumstances, it was as good as ever. A week's exposure in a 

 warm-air cupboard of a piece that did not insulate, made it much 

 better. A film on the outside became non-conducting; but if two 

 fresh surfaces were exposed by cutting, and these were brought into 

 contact with the electrometer and the finger, the inside portion was 

 still found to conduct. 



If the gutta-percha, in either the good or the bad condition (as to elec- 

 trical service), be submitted to a gradually increasing temperature, at 

 about 350 or 380, it gives off a considerable portion of water; being 

 then cooled, the substance which remains has the general properties 

 of gutta-percha, and insulates well. The original gum is probably 

 complicated, being a mixture of several things; and whether the 

 water has existed in the substance as a hydrate, or is the result of a 

 deeper change of one part or another of the gum, Dr. Faraday is not 

 prepared to say. 



ELECTRICITY OF THE HUMAN FRAME. 



AT a meeting of the Paris Academy of Sciences. May 21st, M. 

 de Humboldt sent an extract of a letter, in which M. Emile du Boys- 

 Reyniond describes summarily an experiment, which consists in 

 causing the deviation of the needle of a galvanometer by the effect 

 of muscular action. He takes a very sensitive galvanometer, and 

 fixes at its extremities two slips of perfectly homogeneous platina 

 these two slips he plunges into two vessels filled with salt water, and 

 introduces into them two corresponding fingers of his two hands. 

 At the first immersion of the fingers a more or less decided devi- 

 ation of the needle is always produced, the direction of which fol- 

 lows no law, and which is probably due, at least in part, to some 

 heterogeneousness of the skin of the fingers. When there is a 

 wound on one of the fingers the deviation is stronger, and is always 

 directed in such a way as to show that the wounded finger behaves as 

 the zinc of a zinc-copper couple, supposed to be between the vessels, 

 in place of the body. Of course this is not the kind of action we are 

 concerned with now ; on the contrary, in order to observe the effects 

 announced, we must wait either until the needle has returned to the 

 zero of the scale, or until it has taken a steady position under the con- 



