^&^\ Analysis of Scientific Books and Memoirs. , 



ther with the greater or less velocity with which it is projected ; and hence, 

 according to the state of the surflice, we shall have all the different co- 

 lours." P. 105. 



Now, if we admit that the fact of the production of colour on the steel 

 is correctly stated, we cannot, for our part, see in all this the slightest 

 glimpse of an explanation. The explanation is in reality a mass of hypo- 

 theses, far more difficult to understand than the fact itself; and we can- 

 not conceive, how a man of sound judgment could allow his pen to record 

 such unmeaning extravagancies. But, independent of this, the fact is 

 erroneously described ; and where it is correctly stated, the author's as- 

 sumptions, even if he is allowed the use of them in all their generality, 

 have no reference to it whatever. 



The colours of the steel are assumed to be occasioned by an increase of 

 temperature. But this is not the case ; for if the steel is heated ovt of con- 

 tact with air, no colours are produced ; so that Mr Exley's explanation is 

 completely overturned by this fact alone. 



Again, if the steel surface is examined by homogeneous in place of com- 

 pound light, it will be found, that a state of the surface, produced at the 

 temperature of 500°, reflects the yellow rays copiously ; and a state of the 

 surface, produced at a temperature of 650°, also reflects the yellow rays co- 

 piously ; while a state of the surface, at an intermediate temperature of 

 570°, will reflect no yellow hght at all. But, according to xMr Exley, it 

 ought to reflect yellow at only one state of the surface. We may add 

 another fact, equally baffling to Mr Exley's hypotheses. Signer Fusinieri 

 has found, that these colours are produced on all metals except platinum. 



We shall now request our readers to compare the phenomenon and ex- 

 planation of it, as given by Mr Exley, with the following simple state- 

 ment : — 



When a polished steel surface is heated in contact with oxygen, an oxide 

 is formed in the state of a thin pellicle upon its surface. At a temperature 

 of 430° the colour of the pellicle is straw yellow. At 500° it is a brown- 

 ish yellow. At 550° it is a dark purple- At 570° it is a deep blue. At 

 630° it is a pale blue, with a tinge of green. In virtue of what law of 

 affinity the metal combines with the oxygen in the case of steel, and not 

 in that of platinum, we cannot tell, nor can Mr Exley j but we can 

 prove by direct experiment that the pellicle produced at different tempera- 

 tures has different thicknesses, and that the phenomenon of colour is simply 

 a case of thin plates, the colours arising from the same cause as those of 

 the soap bubble, or (to take a better example), of a thin film of fluid laid 

 upon a steel surface. Chemistry can alone explain the chemical part of the 

 phenomenon ; and the present theory of recurrent colours affords a complete 

 solution of the optical part of it. 



We have thus analysed one of the many hundred phenomena which oc- 

 cupy Mr Exley's pages, and we are confident that he will himself see the 

 incorrectness of his own views, and acknowledge the imj)ossibility, that any 

 one man should be capable of unveiling the mysteries of the natural world 

 by such summary processes as those which we have been considering.. 

 Five hundred able men might execute a work like the present, and the re- 



