146 THE ART OF PROJECTING. 



exhaust, when a dense cloud will be formed ; then on 

 blowing into the flask the cloud disappears. The cloud 

 may be produced and dissolved as often as wished, 

 and if a beam from the oxyhydrogen light be sent 

 through the flask the experiment becomes very effec- 

 tive. C. y. Woodward in Nature. 



MAXIMUM DENSITY OF WATER. 



Take a small test-tube, not more than 

 two or three inches long and half an inch 

 in diameter, and through a tight-fitting 

 cork thrust a small glass tube about three 

 inches long, allowing it to project as 

 much as two inches. Fill the test-tube 

 with water at about 4 centigrade and 

 cork it tight, so that the water will rise in 

 the glass tube. See that there are no air 

 bubbles beneath the cork. Mark the 

 height of the water in the small tube by 

 tying a thread about it. Project the 

 whole with a lantern or with the porte 

 lumiere. Now, if a small vessel contain- 

 1O9. ing hot water be brought up under the 

 test-tube so that the latter dips in it, the expansion of 

 the water will be indicated by the rise of the water in 

 the tube, and the latter will overflow if it be sufficiently 

 heated. No\v, bring up under it in the same way a 

 freezing mixture of ice and salt, or a mixture of equal 

 parts of cold water and nitrate of ammonium. The 

 water will contract in volume to its minimum, which 

 should be indicated by the thread ; then it will again 

 expand until it freezes, the expansion again causing the 

 water in the tube to overflow. The freezing mixture 

 should be stirred constantly to hasten the work. 



