674 POPULAR SCIENCE MONTHLY. 



twenty- four thousand parts of thoroughly dried clay, and was 

 very uniformly distributed. 



The report says : " In order to calculate, with some accuracy, 

 the value of this body of wealth, we cut out blocks of the clay, 

 and found, on an average, a cubic foot, as it lies in the ground, 

 weighs one hundred and twenty pounds, as near as may be. The 

 assay gives seven-tenths grain, say three cents' worth, to the cubic 

 foot. Assuming the data already given, we get four thousand 

 one hundred and eighty million cubic feet of clay under our 

 streets and houses, in which securely lies one hundred and 

 twenty-six million dollars. And if, as is pretty certain, the cor- 

 porate limits of the city would afford eight times this bulk of 

 clay, we have more gold than has yet been brought, according 

 to the statistics, from California and Australia." 



Other calculations show that every time a load of clay is 

 hauled out of a cellar enough gold goes with it to pay for the 

 carting; and if the bricks which front our houses could have 

 brought to their surface, in the form of gold leaf, the amount of 

 gold which they contain, we should have a glittering show of two 

 square inches on each brick. A single specimen of zinc proved 

 to be absolutely free from gold. 



These investigations proved that, while gold is justly consid- 

 ered one of the rarest metals, it is also one of the most widely 

 diffused, and there are many philosophical reasons to be found in 

 explanation of this apparent paradox. 



THE FORCES IN AN AIR BUBBLE. 



BY M. G. VAN DEB MENSBBUGGHE.* 



IN 1880 I had the honor of lecturing before the Class of Science 

 on the metamorphoses undergone by a drop of water, when I 

 described the several phases of the grand cycle which the drop 

 passes through from the moment it forms part of the great ocean 

 mass till the time when after long journeys and numerous trans- 

 formations it again joins its companions in the sea. A few 

 months ago I in a similar way told the history of a grain of dust, 

 dwelling especially on the universality and abundance of solid 

 particles in the atmosphere, pervading everywhere on the surface 

 of the earth. 



I now proceed to describe the career of another minute body 

 hundreds of times lighter than a drop of water or a solid cor- 

 puscle, confining myself to the consideration of its relations with 



* Address before a public meeting of the Belgian Academy of Sciences, December 14, 

 1895. 



