286 
Progress in Science . 
[April, 
cajeput * for an hour : at the end of this time the oil is removed, the sections 
drained on blotting-paper and immersed in turpentine : they are then ready 
for mounting in balsam or damar. Oil of cajeput is preferred to oil of cloves 
as being more limpid, much cheaper, and not giving its own colour to the 
tissue : it is freely miscible with spirit and turpentine, in all proportions. 
Electricity. — Dr. Kerr, of Glasgow, has discovered a new relation between 
electricity and light. He finds that when plate glass is intensely dieleCtrified, 
and traversed by polarised light in a direction perpendicular to the lines of 
force, it exerts a partially depolarising aCtion upon the light, giving an effeCt 
which is much more than merely sensible in a common polariscope. There is 
a good regular effeCt when the plane of polarisation is at 45 0 to the lines of 
force ; no regular effeCt when the plane of polarisation is parallel or perpendi- 
cular to the lines of force. EleCtric force and optical effeCt increase together. 
The optical effeCt of a constant eleCtric aCtion takes a certain time to reach its 
full intensity, which it does by continuous increase from zero ; and it falls 
again slowly to zero after the eleCtric force has vanished. The dieleCtrisation 
of plate glass is equivalent optically to a compression of the glass along the 
lines of eleCtric force. DieleCtrified glass aCts upon transmitted light as a 
negative uniaxal with its axis parallel to the lines of force. DieleCtrified 
quartz, like glass, aCts upon transmitted light as if compressed along the lines 
of force, while dieleCtrified resin, unlike glass, aCts as if extended along the 
lines of force. Carbon disulphide is birefringent when dieleCtrified, aCting 
upon transmitted light as glass extended along the lines of force. The electro- 
static force and the birefringent power increase together ; they also vanish 
simultaneously, the optical effeCt disappearing abruptly and totally at the 
instant of eleCtric discharge. Of the liquids examined there are six which 
have given definite and constant results, namely, carbon disulphide, benzol, 
paraffin and kerosene oils, oil of turpentine, and olive oil. These bodies are 
distinctly birefringent when dieleCtrified, aCting upon transmitted light as 
uniaxal crystals with axes directed along the lines of force, the uniaxal being 
negative in the case of olive-oil, positive in the other five cases. DieleCtrified 
olive-oil aCts in the same way as dieleCtrified glass, or as glass compressed 
along the lines of force ; the other five liquids dieleCtrified aCt as resin dieleC- 
trified, or as glass extended along the lines of force. Compared among them- 
selves, with reference to strength of birefringent aCtion, the liquids appear to 
be very unequal — carbon disulphide the strongest, paraffin and kerosene the 
weakest. Compared with glass they are much weaker insulators ; but if al- 
lowance be made for this difference, for intensity as well as purity of effeCts, 
carbon disulphide is far superior to glass. In contrast with glass, all the 
liquids are characterised by the absence of coercive force, and by the rapidity 
of variation of birefringent aCtion from point to point of the eleCtric field. 
The birefringent power is sustained in liquids by the present aCtion of eleCtric 
force at each instant : it seems also to be determined at each point, simply by 
direction and intensity of force at the point. The author enunciates the three 
following assumptions : — (1.) The particles of dieleCtrified bodies tend to ar- 
range themselves in files along the lines of force. (2.) Changes of molecular 
arrangement, consequent upon rise or fall of eleCtric aCtion, are effected slowly 
and with difficulty in solids, easily and at once in liquids. (3.) The lines of 
eleCtric force, or the axes of molecular files, are lines of compression in one 
class of dielectrics (glass, &c.), and lines of extension in another class (carbon 
disulphide, &c.). The faCts, when thus interpreted, afford a strong confirma- 
tion of Faraday’s theory of electrostatic induction ; and in whatever way 
interpreted they give promise of some new insight into that interesting subject, 
the molecular mechanism of eleCtric aCtion. 
Technology. — In his “ Report on the Development of the Chemical Arts 
during the last Ten Years,” Dr. Hofmann says that decolorisation is effected 
* Distilled from the leaves of Melaleuca minor , a plant growing in the Molucca Islands. 
