350 PRESERVATION OF MARINE ORGANISMS 



of marine animals, it is necessary to make quite clear 

 the difference between formaldehyde 10 per cent, and 

 formalin 10 per cent. Formalin, as purchased, con- 

 tains a definite quantity of the gaseous formaldehyde 

 dissolved in water, and the percentage of formalde- 

 hyde present should be stated on the manufacturer's 

 label thus : " Formaldehyde 40 per cent." To make 

 up a solution so as to contain 10 per cent, of formalde- 

 hyde we must add three parts of water to the quantity 

 taken out of the 40 per cent, stock bottle (100 c.c. of 

 formaldehyde 40 per cent. + 300 c.c. of water). But 

 if we make up a solution to contain 10 per cent, of 

 formalin, then to the quantity taken out of the bottle 

 labelled " Formaldehyde 40 per cent." we add nine 

 parts of water (100 c.c. of formaldehyde 40 per cent.+ 

 900 c.c. of water). This 10 per cent, solution of 

 formalin is equivalent to formaldehyde 4 per cent., 

 and formaldehyde 10 per cent, is two and a half times 

 stronger than formalin 10 per cent. 



Both methods of making up working solutions are 

 in use ; and as authors frequently use the terms 

 " formaldehyde " and " formalin " indiscriminately, 

 without giving a clue as to whether the percentages 

 refer to formaldehyde or formalin, there is consequently 

 much confusion. 



There can be no doubt that the scientific method is 

 to reckon the percentage in the quantity of formalde- 

 hyde present, especially as the commercial solutions 

 of formaldehyde now vary from 30 to 40 per cent. 



Formaldehyde 10 per cent, is a constant strength, 

 whereas formalin 10 per cent, is liable to vary in the 

 amount of formaldehyde present, according to the 

 strength of stock solution. 



The unscientific method of making solutions pre- 

 vails, partly because all commercial solutions were sold 



