48 Things not generally Known. 



a surface capable of being printed, is in the above process 

 done by the cooperation of Electricity with Photography. This 

 invention of M. Pretsch, of Vienna, differs from all other 

 attempts for the same purpose in not operating upon the pho- 

 tographic tablet itself, and by discarding the usual means of 

 varnishes and bitings-in. The process is simply this : A glass 

 tablet is coated with gelatine diluted till it forms a jelly, and 

 containing bi-chromate of potash, nitrate of silver, and iodide 

 of potassium. Upon this, when dry, is placed face downwards 

 a paper positive, through which the light, being allowed to 

 fall, leaves upon the gelatine a representation of the print. It 

 is then soaked in water; and while the parts acted upon by the 

 light are comparatively unaffected by the fluid, the remainder 

 of the jelly swells, and rising above the general surface, gives 

 a picture in relief, resembling an ordinary engraving upon 

 wood. Of this intaglio a cast is now taken in gutta-percha, 

 to which the electro process in copper being applied, a plate 

 or matrix is produced, bearing on it an exact repetition of 

 the original positive picture. All that now remains to be done 

 is to repeat the electro process ; and the result is a copper-plate 

 in the necessary relievo, of which it has been said nature fur- 

 nished the materials and science the artist, the inferior work- 

 man being only needed to roll it through the press. Quarterly 

 Review, No. 202. 



SCIENCE OF THE SOAP-BUBBLE. 



Few of the minor ingenuities of mankind have amused so 

 many individuals as the blowing of bubbles with soap -lather 

 from the bowl of a tobacco-pipe ; yet how few who in child- 

 hood's careless hours have thus amused themselves, have in 

 after-life become acquainted with the beautiful phenomena of 

 light which the soap-bubble will enable us to illustrate ! 



Usually the bubble is formed within the bowl of a tobacco- 

 pipe, and so inflated by blowing through the stern. It is also 

 produced by introducing a capillary tube under the surface of 

 soapy water, and so raising a bubble, which may be inflated to 

 any convenient size. It is then guarded with a glass cover, to 

 prevent its bursting by currents of air, evaporation, and other 

 causes. 



When the bubble is first blown, its form is elliptical, into 

 which it is drawn by its gravity being resisted ; but the instant 

 it is detached from the pipe, and allowed to float in air, it be- 

 comes a perfect sphere, since the air within presses equally in 

 all directions. There is also a strong cohesive attraction in 

 the particles of soap and water, after having been forcibly dis- 

 tended ; and as a sphere or globe possesses less surface than 

 any other figure of equal capacity, it is of all forms the best 



