134 VESTIGES OF THE 



highly polished surface, beautifully iridescent, and, when 

 broken, a foliated texture. The examination of it has 

 even thrown some light on the character and mode of 

 formation of natural shell. "The plates into which the 

 substance is divisible have been formed in succession, and 

 certain intervals of time have elapsed between their for- 

 mation ; in general, every two contiguous lamime are sepa- 

 rated by a thin iridescent film, varying from the three to 

 the fifty millionth part of an inch in thickness, and produc- 

 ing all the various colours of thin plates which correspond 

 to intermediate thicknesses : between some of the lamina3 

 no such film exists, probably in consequence of the 

 interval of time between their formation being too short ; 

 and between others the film has been formed of unequal 

 thickness. Theie can be no doubt that these iridescent 

 films are formed when the dash- wheel is at rest during 

 the night, and that when no film exists between two 

 lamina^, an interval too short for its formation (arising, 

 perhaps, from the stopping of the work during the day)^ 

 has elapsed during the drying or induration of one 

 lamina and the deposition of another."* From this it 

 has been deduced, by a patient investigation, that those 

 colours of mother-of-pearl, which are incommunicable to 

 wax, arise from iridescent films deposited between the 

 laminre of its structure, and it is hence inferred that th<i 

 animal^ like the wheel, rests iKrwdlcalbj from Its labours 

 informin(j the 'natural substance. 



These, it will be owned, are curious and not irrelevant 

 facts ; but it will be asked what actual experience says i-e- 

 specting the origination of life. Are there, it will be said, 

 any authentic instances of either plants or animals, of how- 

 ever humble and simple a kind, having come into existence 



* Mr. Leoiiunl Horner and Sir David Brewster, on a substanco 

 resembling shell. — Philosophkal Train^avtions, 1836. 



