[7] FISH CULTURE IN CENTRAL EUROPE. 545 



is a compound of aluminium, iron, and carbon, the preparation of which 

 is kept a secret: it is used largely in the British navy), which, even 

 after having- served for a long time, will not leave in the water which 

 passes through it any traces of ammonia or any spores, which does not 

 happen when charcoal is employed. In my opinion the use of carferal 

 for filtering the water to be employed in hatching fish eggs would at 

 least keep away the mold ; and perhaps it would be possible to use 

 water containing a large quantity of organic substance of another na- 

 ture. 



To reach a conclusion in this matter I should say that under certain 

 circumstances filtration may be unnecessary, especially if the sediment 

 is so light that it remains floating in the water; but it will be neces- 

 sary, if the sediment forms deposits on any bodies submerged in the 

 water, or if, owing to its peculiar nature, it possesses injurious qualities. 

 But filtration may be recommended under all circumstances, if for no 

 other reason, because one would rather see the eggs clean, and also be- 

 cause inspection would become easier. 



Aeration of the icater. — A defect which is sometimes found in spring 

 water, especially if it flows into the hatching-box after having for some 

 time passed through closed canals, is the scarcity of air, which is not 

 the case in brook or river water which has for a long distance passed 

 through the open air, and which through its constant contact with it 

 has retained a large supply of this vivifying agent. 



Some fish-culturists, as, for instance, Mr. Schuster, consider the aera- 

 tion of the water of such importance as to favor it and to increase it 

 artificially, when there does not seem to be a sufficient quantity of air 

 in the water. I cannot but think that artificial aeration of the water 

 is absolutely necessary in cases where the air is lacking, and is a laud- 

 able precaution when such lack of air is suspected, though there may 

 be no means of proving it, aud superfluous when water is used which 

 contains a superabundance of air; but under no circumstances will 

 aeration prove hurtful. Moreover, the different kinds of apparatus 

 used for artificially aerating water are so simple and so little expensive 

 that economical reasons should never prevent people from using them. 



I am sorry that I have lost the design of the air-injector of Mr. 

 Schuster which I saw in position at the mouth of the outflow of the 

 water into the hatching troughs, but I will give a description of the 

 principle on which it is based. It simply consists of two concentric 

 metal vessels, into the lower one of which the water flows from the 

 other through holes in the bottom. The water in forcing itself through 

 these holes produces air, which enters at the upper part of the central 

 tube and mingles with the water. 



At Keuhausen the water destined for hatching is aerated by means of 

 pipes which carry it underneath the hatching-house. These pipes are 

 placed near the surface of the water in distributing canals, and are of 

 S. Mis. 90 35 



