LIMITING CONDITIONS. 441 



and as there are various forces, and an almost unlimited 

 number of different substances, and an almost infinite 

 series of temperatures, the effects of guiding conditions 

 are numberless. 



A limiting or regulating condition is one which deter- 

 mines the limit of effect in the particular case : for 

 instance, taps, cocks, and valves limit the height and flow 

 of water, the exit of steam, &c. ; the governor and fly- 

 wheel of an engine limit and regulate the velocity. The 

 atomic weights of the elementary substances are limiting 

 conditions of the proportions in which those substances 

 can chemically combine ; the degree of atmospheric pres- 

 sure is a limiting condition of the boiling-points of 

 iquids ; and so on. All natural phenomena are limited 

 n magnitude by one at least of the conditions present : 

 a chain is no stronger than its weakest link ; also, when 

 force produces an effect, it always produces different 

 degrees of that effect in different cases ; and this difference 

 >f degree depends not only upon the intensity and amount 

 >f energy, but also upon the extent to which the limiting 

 conditions allow it to operate. 



An obstructive condition consists either of an opposing 

 cause or a static resisting condition (such as inertia of the 

 mass or of the molecules), which retards or partly pre- 

 vents an effect. The presence of arsenic in a copper tele- 

 graph wire is an obstructive condition to the passage 

 of an electric current. A permitting condition is the 

 reverse of a preventive one; absence of moisture is a 

 permitting condition, its presence a preventive one, and 

 a high temperature an accelerating one, of combustion ; 

 substances burn more rapidly if they are previously dried 

 and heated. A very low temperature is, on the other 

 hand, nearly always a preventive of chemical union. A 

 preventive condition is one which is incompatible with the 



