24 



MATTER AND ITS PHYSICAL PROPERTIES. 



platinum wire is extended along the axis of a cylindrical mould. Into this , 

 mould, molten silver is poured. Since the heat necessary for the fusion of pla- < 

 tinum is much greater than that which retains silver in the liquid form, the 

 platinum wire remains solid, while the mould is rilled with the silver. When 

 the metal has become solid by being cooled, and has been removed from the 

 mould, a cylindrical bar of silver is obtained, having a platinum wire in its 

 axis. This bar is then wiredrawn, by forcing it successively through holes 

 diminishing in magnitude, the first hole being a little less than the wire at the 

 beginning of the process. By these means, the platinum is wiredrawn at the 

 same time and in the same proportion with the silver ; so that whatever be the 

 original proportion of the thickness of the platinum wire to that of the mould, 

 the same will be the proportion of the platinum wire to all the successive thick- 

 nesses to which it is reduced. If we suppose the mould to be ten times the 

 thickness of the platinum wire, then the silver wire throughout the whole pro- 

 cess will be ten times the thickness of the platinum wire which it includes 

 within it. The silver wire may be drawn to a thickness not exceeding the 

 three hundredth of an inch. The platinum will thus not exceed the three 

 thousandth of an inch. 



It now remains to disengage this fine filament of platinum from the surround- 

 ing silver. For this purpose, the wire is bent into the form of a loop, as re 

 resented in the figure, with hooks at A B for suspending it. The part C D 



D 



is now immersed in nitric acic. by which the silver is dissolved, and the pla- 

 tinum remains suspended in a thread so fine as to be invisible without the aid 

 of the microscope. 



By this method, Dr. Wollaston succeeded in obtaining wire the diameter of 

 which did not exceed the eighteen thousandth of an inch. A quantity of this 

 wire, equal in bulk to a common die used in games of chance, would extend 

 from New York to New Orleans. 



Newton succeeded in determining the thickness of very thin lamina of 

 transparent substances by observing the colors which they reflect. A soap- 

 bubble is a thin shell of water, and is observed to reflect different colors from 

 different parts of its surface. Immediately before the bubble bursts, a black 

 spot may be observed near the top. At this part the thickness has been proved 

 not to exceed the two million five hundred thousandth of an inch. 



The transparent wings of certain insects are so attenuated in their structure, 

 that fifty thousand of them placed over each other would not form a pile a quar- 

 ter of an inch in height. 



Jn the manufacture of embroidery it is necessary to obtain very fine gilt sil- 

 ver threads. To accomplish this, a cylindrical bar of silver, weighing three 

 hundred and sixty ounces, is covered with about two ounces of gold. This gilt 



