MECHANICS AND USEFUL ARTS. 83 



ON TUE ESTIMATION OF WEIGHTS OF VERY SMAXL PORTIONS OF 



MATTER : BY PROF. McMAYER. 



The chemist, in the course of his analytical investigations, often meets 

 with what are called traces of substances ; by which is generally understood* 

 quantities of matter too minute to have any appreciable weight in the analyt- 

 ical balance. Now it sometimes happens that these traces are of as much 

 importance, considered scientifically and commercially, as the ingredients 

 present in appreciable quantities ; and in order to estimate these small por- 

 tions of matter, he is often obliged to go over his work, using very consider- 

 able weights of substances, whereby his time and care are nearly doubled. 

 It was this inconvenience that first induced me to try to determine in one 

 operation the components present in large and in very minute quantities; 

 and although I have succeeded beyond my expectations, I am confident that 

 the process is susceptible of improvement, both as regards sensibility and 

 accuracy. 



After making many investigations on the sensibility of the most delicate 

 levers as to small weights, this method was found far too rough. It then 

 occurred to me that if instead of using the opposing force of gravity through 

 the intervention of a lever, we could oppose to the gravitating effect of the 

 matter the force of perfect elasticity as manifested in filaments of glass, we 

 might succeed in obtaining the weights of extremely small parts of matter. 

 For that purpose I tried the elasticities both of torsion and flexure, and 

 found the latter only to answer the purpose. 



The following is a description of the construction of my apparatus, with 

 which I have succeeded in estimating portions of matter equal in weight to 

 the thousandth part of a milligram. Heating a rod of soft glass in one 

 spot to bright redness, I drew it out quickly, and thereby obtained a filament 

 uniformly cylindrical, of about the diameter of fine human hair. Taking 

 from the middle of this fine glass thread a piece of such a length (about 

 three inches) that its weight would barely reduce it from the horizontal, one 

 end of it was fastened, by means of good sealing-wax, to the edge of a ma- 

 hogany block, and the other end slightly hooked by approaching quickly a 

 small spirit flame. In order to obtain a pan in which to place the substance 

 whose weight I would estimate, I cut with the common microscopic section- 

 cutter some discs of elder pith from '001 to '002 inch in thickness; and 

 drawing out a still finer filament, the end was likewise hooked, and the other 

 extremity being passed through a pith disc, a small knob of glass was made 

 on this end by the spirit flame, just of sufficient size to prevent this disc 

 slipping off the suspcnding-rod. The filament with attached disc was now 

 hooked on the end of the rod fixed to the block, and was then ready for 

 graduation. 



Not being able at the time to procure silver wire of sufficient fineness, I 

 substituted some very fine and long hair, taken from the head of a child ; 

 and having brought the centre of gravity and centre of motion of a very 

 sensitive analytical balance almost to coincide, I obtained a piece of the 

 middle of a hair weighing exactly one-half milligram. This being divided 

 into five equal parts (each about one inch long) gave us tenths of a milli- 

 gram. One of these tenths being placed on the pith-pan, the glass filament 

 was deflected a certain quantity, which was marked on an arc formed of 

 bristol board, and so as to be almost touched by the deflected rod in its 



