THE LIME-LIGHT 53 



the larger bore, the distance of the lime must also be in- 

 creased, to get the best result ; so that whereas a small bore 

 may do best with the lime almost close to the metal, a large 

 bore may require a clear space of an eighth of an inch (or a 

 quarter of an inch measured in the oblique line of the jet) 

 from the centre of the orifice to the lime. 



The first step to safety was therefore to store the two gases 

 in separate bags, when the quantities could be adjusted 

 according to the light itself. The mixture ,at the back of the 

 nipple became, while the light was better, far less explosive ; 

 so that a moderate speed of exit would prevent a ' pass-back ' 

 of the flame. Explosions at once became rarer, and confined 

 to accidents or carelessness. For instance, it was usual in 

 those expensive days of chemicals, to keep unused gas in the 

 bags ; and if any bag should be filled up with the wrong gas, 

 it is easy to fancy the result. Even recently a fatal explo- 

 sion at a theatre was traced to this cause. A mistake of this 

 kind is inexcusable ; and if the reader cannot trust himself 

 never to make such, he should not meddle with oxygen in 

 bags at all. The two bags may be of different colours, or, if 

 both are black, the tap of the hydrogen-bag should be 

 coloured, or a large H and should be painted on the bags. 



Another danger was found from the liability to unequal 

 pressure on two bags, as it was usual to place each in a pair 

 of pressure-boards like fig. 26. Fire-irons and fenders were 

 often used as weights, or a lad might be asked to sit upon the 

 top board. Hence there was a danger of the gas under 

 greatest pressure, sent to the nipple faster than it could 

 escape, being driven back so as to gradually mix with the 

 other gas under less pressure. Every alteration of the weight 

 on a bag was thus a precarious experiment, and occasional 

 explosions were practical proofs of the necessity to start with 

 the bags about equally full, and under equal pressure, and to 

 keep them so to the end. For the pressure of gas differs a 

 great deal as the bag gradually discharges, and it is of im- 



