CHEMICAL HASIS OF THE ANIMAL BODY. 



ing power of a given thickness of tin- >uh>tanc.- is determined once 

 for all for a given region of tin- >pectrtnn under ^iven conditions, it 

 becomes possible to determine tlit- amount .f that >uh-tance in any 



solution <>f unknown concentration bj examining the solution under 



the ,-ame conditions in the same part of tin- spectrum and ascertaining 

 liu\v iinich li.u'ht it ha> absorbed. Li-t / In- t In- inten.-it \ <>f the inci- 

 dent light, and 1' its reduced inteii>ity al't.-r pain;_' through in 1 

 .I a coloured solution, each of which reduces the initial intercity by 



! . tli.-n it follows that /' = . 



n m 



This is triii- whatever be the intensity of tin- incident ray; hence tin- 

 intensity may be taken = 1, and we have. /' = 



Again, let E denote the reciprocal of the number which represents 



in centimeters that thickne>s of layer of the ab>orbiiig solution which 

 reduces tin- inten>itv of the incident ray t" ,' of its initial intensity 

 during it- pa->ai,'e through this layer. 1 Then if the solution be exam- 

 ined in a layer which is always 1 cm. thick, this layer may be regarded 



a> made up of E layers, each of thickne-> cm. Hence if in tin- 

 formula previously given we put n = 10 and m = E, we find that 

 the roidual intensity 1' of light after passing through a layer 1 cm. 



thi.-k .- I> = W* = 10 ~* 



\\hence E= log. /'. 



It can also be proved that E, the ( ffirient of extinction. i> 



directly proportional to the amount of colmirin^ matter present in 



the eolation, or in other wordfl, t. it- 'concentration;' 1 whence if the 



ntration be represented by C, 



n 



K = some constant 3 A, or C = AE. 



This constant A having been determined om-e for all for a given 

 substance in ;i solution i,f Known concentration and for 

 /' //'. ni, the concentration of any solution of the same sub 



stance of unknown strength i- olitaim-il l>y simply multiplying A by 



the coefliciellt of extinction / 



Spectrophot, meters an- instruments by which the value \ / 

 ab,,\e) and hence of E may be detennine<|. Those of Yierordt 6 and 



i / bedbd -I-.M-IH. -i.-nt <>f \tinrtiuu. ' a term ininxlu. -,! l.\ IJunacn and Kotcoe, 

 Pogp. Aini.il. 11.1. . i (IV.7). S 



( ii.-i-iitr:itiii ' i- tin- 1111111!.. r of grams of i ..Inuring subtance diwolred 

 in I r i- "( tluiil ( Vii-ronlt). 



; Ciilli-.l tin- ' :il>t.ir|>ti>in r.c 



4 Tin- ititriMliirtiuti f tin- |pactr<i|ihotomftrir nicth.Hl in a r.-li.ilil.- f<nn i 

 ! N'i.-r.T.li. l.:i.-.-.l ii|.-.n tli- |>li..t... li.-!ni.-:il roearche* of Mun-i-n ami I 

 \'i.T"plt. (i) " Anw.-ii-l. 'I S|M-.-tr;ilapparaU /.nr I'li.ii.iin.-trii- .1 A [Kjctren 



11 /. .|ii.uii rlii-in An.il ." TiilHiiu'-n. I H "-'. and (ii) "Die quajit. S|>cctralaaal. in 

 ilir.T Anw.-ri.l auf l'liv.-i.,l. u. > w ." Tubingen, 1876. 



loc.cit. (i). > 



16 



