490 Prof. F. Clowes. [Feb. 2, 



maintained at that height sufficiently long for the completion 

 of the test ; whereas other testing flames are constantly varying 

 in dimensions, and most of them cannot be set to standard size 

 at all with any certainty. 



Thus a colza-petroleum flame, exposed in air containing a 

 low percentage of gas, when twice adjusted, gave caps of 8 and 

 of 20 mm. The reduced oil-flame also fell so quickly that 

 cap-readings with low percentages of gas could not be taken 

 at all. 



3. The caps produced over the hydrogen flame are larger than 



those produced by any flame of corresponding size. 



4. The size of the hydrogen flame can therefore be so far reduced 



as to enable it to be used in an ordinary safety -lamp. 



The size of the flame may further be suitably varied, so as 

 to increase or decrease the height of the cap, and thus either 

 increase the delicacy of the test or extend its range. 



5. The standard hydrogen flame shows no trace of mantle or cap in 



air free from gas; it shows only a slender thread above its 

 apex. The colza-petroleum and the benzoline flames show pale 

 mantles in gas-free air, which may be easily mistaken for a 

 small percentage of gas. 



6. The standard hydrogen flame burns vigorously, and is of fair size, 



and therefore cannot be extinguished by accident ; whereas the 

 reduced flames, ordinarily used in testing, burn feebly and are 

 readily lost. 



7. Hydrogen is supplied fairly pure and of practically invariable 



composition; whereas commercial oil and alcohol are apt to 

 vary much in composition, and therefore to give flames whose 

 indications change with the sample of liquid which is being 

 burnt. 



The Precise Measurement of Flame-caps. 



The flame-caps in a long series of observations, of which the results 

 are here stated, were observed in the test-chamber already described 

 (' Boy. Soc. Proc.,' vol. 50, p. 122 and vol. 51, p. 87). This apparatus 

 has been made more convenient for use by slinging the water- tray, 

 which effects the closure of the chamber below, from the bottom of 

 the chamber by four rods which furnish a kind of parallel motion. 

 The tray can thus be pushed back and again brought into position, 

 and be maintained throughout in a horizontal position. It has also 

 been found necessary to introduce a small circular trap-door in the 

 floor of the chamber, and just behind the window. When an oil- lamp 

 is placed over this opening, its flame can be readily adjusted by the 

 "pricker" from outside, without removing the lamp from the 



