222 THE AMERICAN MONTHLY [July, 



twelve times as great. Even in Philadelphia, it was 6.4 per 10.- 

 000. During 1892 Hamburg drank unfiltered water from the 

 Elbe and had 17.975 cases of cholera, 7,611 of which proved 

 fatal. Altona also drank Elbe water, but only after filtration ; 

 and it had 562 cases of cholera. Its population was about one- 

 fourth that of Haml)urg and its cases about one- fourteenth that 

 of Hamburg. Many of those were importations from Hamburg 

 and not of home production. Wandsbeck, a neighboring city, 

 using sand -filtered water from an inland lake, got off" with only 

 64 cases of cholera in its 20,000 inbabitants. In a street divid- 

 ing Hamburg from Altona, the houses on one side were supplied 

 with unfiltered water and had many cholera victims. On the 

 opposite side, the people drank filtered water and mostly es- 

 caped cholera. 



The Lawrence investigations, covering 5 years and including 

 over 11,000 bacterial examinations of water, indicate the practi- 

 cability of constructing filters which will economical remove 

 over 99 per cent of the bacteria contained in the unfiltered water. 

 The exact depth of snnd required has been ascertained and the 

 number of bacteria lodged in each depth of sand, viz: i inch 

 deep, 1,100,000 bacteria per grain ; 1 inch deep, 320,000 ; 1 inch 

 deep, 140,000 ; 2 inches deep, 21,000; 4 inches, 4,000; 6 inches, 

 1,600. Liiwrence now has a filter, covering 21 acres, which 

 yields 5 million gallons of water daily. It is the only one of its 

 kind in America. 



BIOLOGICAL NOTES. 



Staining Living Cells. — A live tadpole immersed in neutral 

 red (1 to 2 300,000) absorbs the color so rapidly as to have some 

 of its tissues stained in 15 minutes ; and in a longer time all the 

 tissues became perfectl}^ red. The color not only penetrates the 

 outer membranes, but would seem to be attracted b}^ tbe cells. 

 This color has been found to possess the greatest affinit}' for 

 live cells of all the kinds of coloring matter. 



DIATOMS. 



To Fasten Arranged Diatoms. — Put a brush containing 

 very weak gum -water at one end of the line of diatoms, when 



i 



