436 Chronicles of Science. [July, 



Father had ordered a strong iron tube to be made. As it was in- 

 tended for an apparatus reqmring a vacuum, it was essential that 

 this tube should be perfectly air-tight; and as he had some doubts 

 about its soundness in this respect, in order to set these at rest, the 

 tube was made red-hot and taken into a dark place, when there 

 was clearly perceived through the iron (which was half a centi- 

 metre thick), a crack inside the tube, and which did not reach to 

 the outer surface. Commenting on this, Dr. Adriani, in the 

 * Chemical News,' says : — " It is rather curious that the fact of the 

 metal above alluded to, to which I have reason to believe that 

 gold may be added, becoming transparent at red heat, should have 

 escaped the notice of scientific men. It requires, however, a good 

 bright red heat ; but the transparency of the metals is evident thus 

 even in daylight, as I know from my own experience while working 

 in an engineering establishment attached to a large sugar refinery 

 now many years ago." 



Mr. Stock, another correspondent of the same paper, suggests 

 what we consider to be the more probable explanation. He says 

 that it is more likely that the crack exhibited itself, because where 

 it existed the metal was thinner than throughout the rest of the 

 tube, and would therefore cool more rapidly, becoming in conse- 

 quence darker coloured, thus showing on the exterior both its 

 extent and direction. 



Electricity. — A very important modification of the magneto- 

 electric machine has been suggested by Mr. Tisley, the very 

 able and intelligent assistant to Mr. Ladd, and has been car- 

 ried out by that instrument maker. It was suggested that 

 if the armature had two wires instead of one, the current of 

 one being sent through a wire surrounding the magnets, their 

 power would be augmented, and a considerable current might be 

 obtained from the other wire available for external work ; or there 

 might be two armatures, one to exalt the power of the magnets, 

 and the other made available for blasting or other purposes. 

 This idea has been carried out as follows :— Two bars of soft iron, 

 measuring 1\ in. x 1\ in x \ in., are each wound round the 

 centre portions with about thirty yards of No. 10 copper wire, and 

 shoes of soft iron so attached at each end that when the bars are 

 placed one above the other, there will be a space left between the 

 opposite shoes in which a Siemens' armature can rotate. On each 

 oi the armatures is wound about ten yards of No. 14 copper wire, 

 cotton-covered. If the armature in connection with the electro- 

 magnet is made to rotate, there will be a very feeble current 

 generated in it ; this passing round, the electro-magnet will increase 

 its power with every additional impulse. It will thus be seen that 

 the only limit to the power of the machine is the rapidity with 



