ILLUMINANTS 11 



occur from a too sudden "jerking on " of the oxygen. To avoid the possibility of 

 this accident, Mr. Beard (and others, we beUeve) have arranged a " check" which is 

 placed in the mouth of the gauge, which only allows the oxygen to pass very sloivly 

 no matter how much the gas bottle valve is turned on. That is to say, even if by 

 accident this latter valve is violently opened an undue amount, the gas only enters 

 the gauge quite slowly. It is an invention which can be added at the small expense 

 of five shillings and is well worth the money. 



So far as relates to limes, the very best "hard" Nottingham variety is always to 

 be bought for the mixed jet, but the ''soft lime" does quite well for the blow-through 



Fig. 7 



Fig. 8 



and is cheaper. Messrs. Zeiss have introduced a small lime which is some compound 

 of Zirconium. We have tried it and like it very much. Ordinary limes rapidly wear 

 away under the intense heat of the mixed jet, and require frequent turning, but these 

 of which we have just spoken are extremely hard and do not apparently require any 

 attention for some protracted interval. Their only fault seems to be their price and 

 that a special arrangement of the jet is required for their use. 



So far as to the use of artificial lights. Another illuminant exists, however, which 

 is almost a necesssity for a few picked specimens— we refer to the sun. This, when 

 employed, requires some form of special arrangement, owing to the rotation of the 



