490 transactions of the american institute. 



Zinc Relief. 



Bocttger described at Giessen, a method of writing on zinc with a solu- 

 tion consisting of one part of chloride of platinum, one part of gum Arabic, 

 and ten parts of water, by which black characters are produced. When 

 the plate is placed in dilute nitric acid, the parts not written upon are 

 eaten away, and the writing stands out in relief. 



On Drops. 



Prof. Guthrie of Mauritius, in an elaborate paper, read before the Koyal 

 Society on drops. After detailing the numerous experiments made by the 

 author, to ascertain the sizes of drops obtained from various liquids under 

 different conditions, he gives the following laws governing a liquid dropping 

 from a solid through a gas. 



1. The drop size depends upon the rate of dropping. Generally the 

 quicker the succession of drops, the greater the drop; the slower the rate 

 the more strictly is this the case. This law depends upon the difference, at 

 different rates, of the thickness of the film from which the drop falls. 



2. The drop size depends upon the nature and quantity of the solid, 

 which the dropping liquid holds in solution. If the liquid stands in no 

 chemical relation to the solid, in general, the drop-size diminishes as the 

 quantity of solid contained in the liquid increases. The cause of this seems 

 to be, that the stubborn cohesion of the liquid is diminished by the solid 

 in solution. When one or more combinations between the liquid and solid 

 are possible, the drop-size depends upon indeterminate data. For example 

 certain varieties of the drop-size of solutions of chloride of calcium of dif- 

 ferent strengths, point to the existence of definite hydrates; while the regu- 

 larity of the variation of drop-size in the case of nitrate of potash, points to 

 the absence of hydrates. 



3. The drop-size depends upon the chemical nature of the dropping liquid, 

 and little or nothing upon its density. Of all liquids examined, water has 

 the greatest, and acetic acid the least drop size. It is remarkable that 

 butyric acid, which has sensible the same specific gravity as water, gives 

 a drop le^s than half the size of the water drop. 



4. The drop-size depends upon the geometric relation between the solid and 

 the liquid. If the solid be spherical, the largest drops fall from the largest 

 spheres. Absolute difference in radii, takes a greater effect upon drops 

 formed from smaller, than upon those formed from larger spheres. Of cir- 

 cular horizontal planes, within certain limits, the size of the drop varies 

 directly with the size of the plane. 



The fact that the drop increases in size according to the increasing radius 

 of the sphere from which the drop falls, and that tlie difference from 

 this cause may amount to half the largest drop-size, the author regards as 

 important to dispensers of medicines. The lip of a bottle from which a 

 drop falls is usually ring-shaped. The amount of solid in contact with the 

 dropping liquid is determined by the size of the two diameters, one measur- 

 ing the width of the rim of the neck, the other its thickness. In most 

 cases the curvature and massing of the solid at the point whence the liquid 

 drops, is so irregular as not to admit of any mathematical expression. 



