564 TRANSACTIONS OF SECTION G. 
designing efficient and graceful structures in metal, but the best results can only 
be attained by a designer who has a thorough scientific and technical knowledge 
of the properties of steel and the processes of its manipulation on the one hand, 
and cultured artistic sense and capacity on the other. These should not be 
considered as appropriate equipments for separate professions. 
There are many, however, who have a rooted conviction that structures 
in steel can never be so beautiful as those in stone. This I believe to be alto- 
gether wrong. It arises partly from the crudity of design that characterises most 
of the steel structures that have yet been erected, and partly from preconceived 
notions as to what is fitting in proportions and massiveness. We can quite 
imagine that a native of the Congo region whose notions of the proportions suit- 
able and comely for a quadruped were founded on his familiarity with the 
hippopotamus would, at first sight, consider the racehorse sadly lacking in sub- 
stance and solidity, but, in time, he might come to recognise some measure of 
gracefulness in a creature that has been developed to meet requirements that 
hitherto he had not fully considered. 
Mr. Wells has said in his ‘New Utopia’: ‘The world still does not dream 
of the things that will be done with thought and steel when the engineer is 
sufficiently educated to be an artist, and the artistic intelligence has been 
quickened to the accomplishment of an engineer.’ But we need not postpone 
till the advent of a complete Utopia the full realisation of our duty to practise 
our profession as far as in us lies, with due regard for the material interests and 
the zsthetic susceptibilities of all who can be affected by the works for which 
we are responsible. 
The following Papers and Report were then read :— 
i. The Ignition of Gaseous Mixtures by Momentary Electric Ares. 
By Professor W. M. Tuornton, D.Sc., D.Eng. 
The inverse of the gas-engine case—that is, under what conditions will single 
electric sparks fail to ignite gas—is of importance in coal-mining. The present 
note is an investigation of the least currents, direct and alternating, which are 
required to ignite gaseous mixtures at different voltages and frequencies. 
The lower and upper limits of mixture within which ignition is possible are 
found to be 425 and 14 per cent. for fire-damp in air, 6 and 40 per cent. for 
coal gas. There is a well-defined maximum of sensitiveness to ignition at 7 per 
cent. in the former, 8°5 per cent. in the latter. With direct voltage the least 
igniting current is approximately proportional to the reciproca] of the voltage; 
with alternating voltage the frequency is of more importance, and the current 
remains constant over a long range of voltage, being higher for methane than for 
coal gas. The energy of spark which will just ignite the most sensitive mix- 
tures is about 0°10 joule, corresponding to the combustion of 37 c.c. of an 11 per 
cent. mixture of coa] gas. Single sparks therefore give relatively poor ignition. 
It would appear from the results that when alternating current is used for 
signalling or where there is no continuous sparking at a contact, the risk from 
electrical signalling in coal mines is extremely small. Where there is sparking, 
as at a vibrating contact, the bells must be enclosed in flame-proof cases, or be 
situated in places which cannot be reached by fire-damp in mixtures approaching 
4 per cent. 
2. Fifth Report on Gaseous Lxplosions.—See Reports, p. 192. 
3. Description of the Home Office Experimental Station for Research 
on Explosions. By Professor H. B. Dixon, F.R.S, 
