SCIENTIFIC SUMMARY. 
559 
ment of positive electricity as pyroxiline paper does. The most energetic 
effects are produced when vulcanised india-rubber is the electric. The 
opposite effects produced in this substance by flannel and gun cotton, or 
pyroxiline paper, are very striking, and will form a good lecture-room 
illustration. These substances also produce powerful positive excitement 
in glass. It is difficult from the use of pith balls alone to determine which 
produces the most powerful positive excitement, glass or hard rubber, when 
excited by gun cotton or pyroxiline paper. This seeming anomaly, con- 
founding our ordinary means of discrimination in cases of electrical 
excitement, demands further investigation. It would appear that, of nega- 
tive electrics yet observed, these azotised species of cellulose are the most 
remarkable, in comparison with which the most highly negative electrics 
hitherto known become positive. 
The new Atlantic Cable . — One of the improvements in this cable is the 
mode in which the gutta-percha coating is applied. Some time ago 
Mr. Chatterton discovered and patented a compound that is said to have 
the property of causing the coatings of gutta-percha to adhere firmly, 
without the injurious use of naphtha, so as to form a completely solid core. 
The wires having been formed into a strand, and six wires twisted round 
a central one, they are passed through a vessel containing Chatterton’s 
compound, and are then covered with their first coat of gutta-percha to a 
depth of about an eighth of an inch. After having been examined by 
hydraulic pressure and by hand, the core receives another coating of 
Chatterton’s compound, and then another layer of gutta-percha, and 
so on till it has attained a diameter of a little more than half an inch. It 
is then covered with the solid wires drawn from Webster’s and Hors- 
fall’s homogeneous iron, each capable of sustaining a strain of 1,000 lb., 
at which strain it only stretches one per cent. These homogeneous 
iron wires are surrounded separately with five strands of Manilla yarn, 
saturated with a preservative compound, the poisonous qualities of which 
have been found by experiment to prevent its destruction by animals ; and 
the whole are laid spirally round the core, which latter is padded with 
ordinary hemp, saturated with preservative mixture. The weight of the 
cable when completed is 25 cwt. per mile, when weighed in air, but its 
specific gravity is said to be so little, that it is capable of bearing eleven 
miles of its length when in water, a strain it is quite impossible it should 
ever be put to, the deepest water encountered, in laying the old cable, 
being less than 2^ nautical miles. The company have made arrangements 
by which they can pay out fifty miles per week of the new cable. — Vide 
The Artizan , June, 1864. 
Improved Diffraction-showing Apparatus . — At the conversazione held in 
University College last June Messrs. Horne & Thornth waite exhibited 
a very beautiful instrument of the above kind. It consisted of a telescope 
mounted horizontally, and having at some distance from it, but exactly 
in its longitudinal axis, a small aperture, through which passed a beam 
of oxy-calcium light. Immediately in front of the object-glass was 
placed a disk of glass, on which were photographed some hundred figures 
of different forms, and all so minute as just to be seen by the naked eye. 
This plate, or disk, was so arranged that, by means of a cogged wheel, 
