196 



the size of the drop, in tapering off the divisor, is of no consequence. 

 But it is otherwise in the case of a gold coin, for example: — in a dou- 

 ble eagle, the difference of one drop of water (ordinarily about a half- 

 grain) in the divisor, would affect the result to the extent of 0.3, 

 which, carried into the fineness, would make a difference of 15 or 

 20 thousandths; and in the case of a half-eagle, the uncertainty of 

 result would be proportionally increased. The question then arose, 

 what fluid, or what modification of water, will afford us a smaller 

 drop? for, as was just observed, a half-grain is, on the average, the 

 smallest weight of clean water that will detach itself by its own 

 weight. Very much depends, of course, upon the size of the aper- 

 ture, in the measure of drops of fluid ; one drop of water, from a large 

 beak, weighed li grains. In the Dispensatory of Drs. Wood and 

 Bache, there is a table of the experimental results of Mr. Durand, 

 showing the number of drops of different liquids equivalent to a flu- 

 idrachm (Page 1405). The differences are very remarkable; dis- 

 tilled water, for instance, being set down at 45 drops, and pure alco- 

 hol at 138 drops. And in our own experiments, the drop of alcohol 

 was about one-third the weight of the drop of water, from the same 

 pipette. This seemed to point to alcohol as a substitute; but there 

 were obvious objections, and a much better vehicle was found in 

 soapy water. 



The best white soap, sold at the shops, is of the same specific gra- 

 vity as water, and its mixture with water makes no change, in that 

 respect. When the mixture is as strong as children use for blowing 

 bubbles (we cannot conveniently give this measure in figures), the 

 cohesion or tenacity of the water is so much weakened that the drop 

 is reduced to one-tenth of a grain. No other fluid makes so small a 

 drop as this. And there is the further advantage, that soapy water, 

 though excellent for making bubbles, is less liable to retain them be- 

 low the surface than pure water. So small a drop, of course, makes 

 the experiment more tedious, and, by using less soap, the size of the 

 drop will bo, in many cases, advantageously increased. 



Some years ago, at the desire of Prof. Henry, experiments were 

 made in the assay-office, to test the comparative tenacity of pure wa- 

 ter and soapy water, by observing how much weight, at a beam, 

 would detach metallic disks held together by those fluids, rosi)cctively. 

 This problem is more readily solved by tlie method just stated; 

 namely, by observing the size of the drop. 



This apparatus has a decided advantage for taking specific gravi- 



