ZOOLOGY AND BOTANY. MICROSCOPY. ETC. 875 



they would be were they subjected to the action of weak acids or 

 alkalies. 



At parts more remote from the electrodes changes also occur 

 which, however, are not so remarkable as those which are induced by 

 the chemical processes above alluded to. The effects, which may be 

 trusted as being really due to electricity, should occur quickly after 

 the passage of the current, and not be limited to the part in the 

 immediate neighbourhood of the electrodes. If the current be allowed 

 to pass for some time, that is to say, for more than a few seconds, 

 through the tissue, the products of electrolysis first extend over the 

 whole surface lying between the electrodes, and then the intensity of 

 the current becomes extraordinarily reduced, frequently indeed to 

 zero, on account of the pole becoming covered with bubbles of gas. 

 On this account the employment of constant currents for microscopic 

 investigation is scarcely to be recommended, for with the closure of 

 even veiy weak currents so violent a development of gas occurs, that 

 but little confidence can be placed in the results that are observed to 

 follow their passage. The amount of electrolysis that occurs with 

 induction currents is much smaller, and they have therefore been 

 most generally employed. The arrangement in which there is a 

 single shock on opening and closing of the current is particularly 

 advantageous. The shocks obtained from a Leyden jar are infinitely 

 superior to the constant currents, because the instantaneity of the 

 shock causes the disturbing influence of the evolution of gas bubbles 

 to be altogether abolished. 



With regard to induction currents, he also points out that on 

 breaking the current, heat is developed in the tissue. If an un- 

 covered drop of blood is under examination with strong ordinary 

 lenses, these become dimmed at the instant of the passage of the 

 current, but after a short period they again become clear. The 

 preparation, however, very soon dries up. It is requisite in such 

 cases to determine what are the effects of the sudden elevation of 

 temperature, and what are those of the electric current alone. 



Strieker's electrodes * are shown in fig. 207. The slide (covered with 

 tin-foil as previously described) is held in position by the electrodes, 

 each of which is insulated by being screwed into an ivory knob let 

 into the stage-plate of the Microscope. The electrodes are connected 

 (with the interposition of a key) with tl)e secondary coil of a Du 

 Bois Keymond induction apparatus. In the woodcut the key is re- 

 presented open. 



Mr. K. T. Lewis states f that when investigating the disruptive 

 effects of the electric spark — more especially with regard to tho 

 peculiar shape of the perforations made by it through various materials 

 — many experiments were carried on upon the stage of the Microscope, 

 and he found that a very simple and convenient method of holding 

 and insulating the terminal wires was to pass each through a small 

 glass tube held by a brass spring-clip, mounted upon a jointed pillar 



• Klein, Burden -Sanderson, Fostor, and Briinton, 'Handbook for tho Phvsio- 

 logiml Laborat-jry,' 1873, p. 17 (I fif?). t J'^'if,'!- Modi., xlii. (iSSf.) p.'v,). 



