74 OPTICAL PROJECTION 



as in fig. 49, which would answer very well with a funnel at 

 the top of the long tube. Care must be taken that the top 

 is air-tight. Into the retort is introduced about half a pound 

 of granulated or scrap zinc, and through the funnel is then 

 poured about a pint of dilute sulphuric acid ; about one part 

 of acid to six or seven of water. This should be previously 

 mixed, to avoid heat in the retort. The gas as it comes over 

 is passed through one wash-bottle ( 42) about half full of 

 water, as in washing oxygen ; but great care must be taken 

 that all atmospheric air is allowed to escape. To ascertain 

 this, a light is applied to the gas, which should take fire ; but 

 to avoid explosion, a piece of rather fine wire gauze should 

 be bent into a kind of loose cap, and held over the end of the 

 delivery tube. When the gas inflames quietly, the bag is 

 connected up as usual, turning on the tap at the same 

 moment. If the evolution of gas fails before the bag is full, 

 a little more acid may be poured down through the funnel : 

 or if done very gradually the acid may be added from the 

 first in this way. Any metallic zinc left may be preserved 

 for the next occasion. In turning off the bag-tap when full, 

 the very next moment remove the delivery-tube ; but be 

 careful it does not discharge near any light. 



Should zinc not be obtainable, clean iron filings, or small 

 nails, may be used instead and in the same way. 



39. Oxygen. Till lately oxygen gas was universally pre- 

 pared by every habitual user for his own requirements, and 

 so much is still made from chlorate of potash that it is 

 necessary to describe the best methods of managing that 

 process. It has been stigmatised as dangerous, but is not 

 at all so when properly conducted. There are certain possible 

 dangers, it is true ; but, guarding against these, the operator 

 may proceed with perfect confidence. 



The only practical method of preparing individual supplies 

 of oxygen consists in heating over a furnace a mixture of 

 potassic chlorate and manganese black oxide ; or in lieu of 



