92 THE MICROSCOPE. June 



a little larg-er than the diameter of the platinum wire, 

 which is now placed in position. The exhaustion being- 

 complete, the narrow tube around the wire is heated, and 

 the g-lass becomes compressed around the wire. When 

 cold the superfluous part of the tube is cut off, and it is 

 then easy to free any required leng-th of the wire from the gflass 

 by crushing- with nippers or on an anvil, after which the 

 seal is rounded off in the blow-pipe flame. In this way 

 minute vacuum tubes like beads, the interior of which is 

 but one-eig-ht of an inch long^ and one-sixteenth of an inch 

 in diameter, can be readily made, and, apart from decora- 

 tive uses of such tubes in chains or festoons, surg-ical uses 

 for minute high-vacuum tubes may possibly be found -in 

 connection with Rontg-en's method of observing-, and re- 

 cording- on sensitive surfaces. — The Optician. 



Purification of Drinking-^A/'ate^ by Means of Filtration. 

 — The rejjort of the Massachusetts State Board of Health 

 for the year 1894 contains some very interesting- and im- 

 portant facts upon all these points. 



For the past seven years the board has maintained an 

 experimental station at Lawrence for the sole emd express 

 purpose of testing- the efficacy of filtration of water to 

 purify it and render it fit for household purposes. The 

 water tested was that of the Merrimac River, which is 

 lined from source to moutli with manufacturing-towns, and 

 which may be taken as a fair sample of river water con- 

 taminated with a considerable amount of org-anic matter. 



The filters were of all sizes and thicknesses, from those 

 a fewfeet square and ten inches in depth to the larg-e filter 

 covering two and one-half acres, throug-h which the water 

 supplied to the city of Lawrence has been filtered since 

 1893. 



Chemical and bacteriological examinations were made 

 weekly and sometimes daily of the water of ingress and 

 eg-ress. Sand of different sizes was used, and the filters 

 were run both intermittently and continuously. 



The result of this careful and painstaking- investig-ation 

 extending- over a number of years and every source of 



