PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 449 



exposed to tlie explosive impulse; and tlioug-h the small quantity of air 

 tliey contained gave back by expansion the compression received, there is 

 little doubt that the cifect on the piston would have been greater if there 

 had been no communication with tliese — that is, if the force had been solely 

 and strictly concentrated on the end of the piston. It was not intended to 

 give any statements respecting the experiments till they were concluded, 

 but on closing the remarks on Compressed Air, the opportunity seemed 

 appropriate fur a brief reference to them. 



Mr. G. Hartlett said there appeared to be some miscalculations about the 

 power of gunpowder in the article read. He believed gun cotton to be 

 more expansive than gunpowder. 



The Manufacture of Sugar. 



The Chairman remarked that this subject had been lately very full^f dis- 

 cussed in its chemical bearings, a report of which will be found in the last 

 volume of our transactions. On the present occasion he hoped more time 

 would be given to explanations of the mechanism used in sugar manufac- 

 tories. He proposed to say a few words on 



SACCIIARIMETRy. 



The saccharometer used to determine the quantity of saccharine matter 

 in a fluid is the common hydrometer having a graduation suited to the 

 purpose. It is however essential to discover the quality or variety of the 

 sugar under examination. This was first successfully accomplished by M. 

 Soleil by means of polarized light. Many inorganic substances have the 

 power of rotating the plane of vibration of the polarized ray. The angle of 

 deviation is in proportion to the length of a column of the same solution of 

 cane sugar, and it follows that by using a column of the same length, any 

 variations in the plane of vibrations must be due to a difference in the 

 structure of the compound. Taking advantage of this fact, Soleil con- 

 structed a glass tube surrounded with a case to exclude the light and 

 closed at both ends with plate glass discs ground to fit water-tight and 

 pressed against the tube by means of screw caps. This tube he completely 

 filled with the saccharine liquid and placed it on supports between two 

 Kichol's prisms, one serving as a polarizer through which the ray passes 

 into the liquid, the other being the eye-piece through which the observa- 

 tion was made, and to this was attached a graduated circle having a ver- 

 nier which moved in a vertical plane. The most exact measurements are 

 made by noting the disappearance of what is known as the transition tint, 

 the purple, because the change of this to the red on one hand or the violet 

 on the other is very sudden. This tint is pi'oduced by interposing quartz 

 3.75 millimeters thick, a double plate is used, one-half being composed of 

 right-handed and the other half of left handed quartz. When the rotary 

 power of the liquid under examination is considerable, an error arises 

 which is obviated by what is called a compensator. In order to determine 

 the strength of a solution, it is necessary that the solution contain only one 

 substance, and that the quantity present be proportioned to the angle of 

 rotation ; that rotation of the red ray be known for one degree of concen- 

 tratiitn. Some substances rotate the plane of polarization to the right, 

 [Am. Inst.] C* 



