484 



NATURE: 



[April 22, 1875 



the ordinary wooden capsule a large cork is substi- 

 tuted. " It is cut across the middle, the necessary holes 

 are bored in it for the tubes, and a conical cavity is cut 

 into each half with a sharp penknife, as shown in Fig. 

 3. Large corks are never quite air-tight ; the whole 

 of the outside should therefore be covered with a layer of 

 sealing-wax one or two millimetres thick ; this is done 

 after the two halves have been glued together and the 

 whole is perfectly dry." Before being glued together, a 

 piece of goldbeater's skin is stretched between the two 

 halves at //. The tubes abc are of glass, the aperture of 



c being about o'4 mm. Here we may observe that, instead 

 of goldbeater's skin or collodion film, which students 

 in general will find difficult to procure, a portion of 

 one of those children's toy balloons made of thin india- 

 rubber may be substituted \i\i\\ great advantage. It 

 should be attached as follows : the edge of the capsule 

 is first glued, and the inflated balloon then pressed on it ; 

 when the glue is dry, the portion that remains attached to 

 the capsule is cut round with a knife ; by this means 

 a tense thin film is strained across the instrument. 



These toy balloons will be] found of frequent service in 

 acoustics. 



The useful little instrument just described will therefore 

 cost little beyond the slight trouble of making it. Neverthe- 

 less, the English editor has permitted a firm of instrument 

 makers to advertise it for half a guinea at the end of the 

 volume as "an indispensable piece of apparatus required 

 by the student of this work." In like manner it is "in- 

 dispensable '' to buy a Barker's mill, the price charged 

 being a guinea, when on p. 201 the student is shown how 

 to make one for twopence. We might quote several 

 other instances from this carelessly inserted advertisement. 

 As a translator Mr. Loewy seems to have done his duty 



I 



Heating effects of the discharge 



well, but we would suggest'the necessity of his exercising 

 a little more editorial care if a second edition of this work 

 is called for. 



In the section on Light there are some capital instruc- 

 tions for making concave and convex mirrors, and for 

 constructing a simple form of spectroscope, which is 

 entirely built up by the student. The manufacture of a 

 bisulphide of carbon prism (employed in this spectro- 

 scope) is always a matter of difficulty. Prof. Weinhold 

 recommends making the body of the prism of a lamp 

 cylinder cut to a v^fedge shape by an ignited pastille ; the 



edges are then ground with emery powder, a hole bored 

 for filling the prism, and the sides of plate-glass (French 

 plate should have been stated) cemented on by a mixture 

 of glue and treacle. 



The accompanying woodcuts indicate two simple 

 arrangements for showing the heating power of the 

 electric discharge. In the one case (Fig. 4) wires, bent 

 as shown in the figure, are insulated by sealing-wax and 

 passed through a cork, in the centre of which is a glass 

 tube allowing a gas jet to issue between the wires, the 



gas being ignited on the discharge from an electrophorus 

 between the points. The other apparatus (Fig. 5) 

 shows that e\-en good conductors are heated by the 

 electric discharge. " A small wide-necked glass bottle is 

 closed by a cork, through which two wires pass and also 

 a glass tube, which is drawn to a point about I'S mm. 

 wide, and bent horizontally. The wires are connected by 

 a long, very narrow strip of tinfoil. The glass being very 

 slightly warmed by holding it in the hand for a moment, 

 a drop of water is brought upon the point of the tube. 



