328 H. MORLAND ON MOUNTING DIATOMACE M. 



off the edges of the drop of gum by strongly moistening it by 

 the breath and mixing the edge with the moisture by means of 

 the stiff bristle ; the gum can be rapidly dried again by drawing 

 the breath through the " breathing tube." The diatoms are 

 arranged on this gum in the manner desired, and afterwards 

 fixed by breathing gently through the tube whilst being 

 observed through the microscope, for fear of carrying on this 

 fixing process too far. In the second place, the balsam is applied 

 cold, and if the diatoms stand up rather high it may be neces- 

 sary to apply a fairish quantity of the balsam, which, in such 

 case, must not be too fluid or it will run out at the edges. 

 Practice is the best guide in this matter. 



My objection to using gum or other fixing cement is that in 

 far too great a proportion of cases the same is visible more or 

 less where it gets hold of the diatoms ; for instance, there may 

 be seven or eight diatoms in position, all of which, with the 

 exception of one, being perfect, but the remaining one is spoilt 

 by the gum showing up on it. As far as I can see there is no 

 certain way of avoiding this difficulty, but it is amenable to 

 judgment in some few cases. I try to avoid it as far as possible 

 by breathing as little as I can venture, with the result that one 

 or more of the diatoms may float away on the application of the 

 balsam. On the other hand, some diatoms can be fixed down 

 with impunity, such as some of the Anlacodisci, Trinacria?, etc., 

 with prominent processes upon which they stand and are fixed 

 down, the body of the valve being quite clear of the gum. Some 

 valves with very fine cellular structure sometimes get some of 

 the cells clogged up with the gum, retaining air in them which 

 cannot get displaced by the balsam in consequence of the in- 

 solubility of the gum in the solvent of the balsam. I have heard 

 of a thin solution of caoutchouc in benzole being employed as a 

 fixing cement, but I have never tried it; with this cement, 

 after the diatoms have been arranged, they are fixed down by 

 means of a gentle heat. Now, as caoutchouc is slowly 

 soluble in benzole, it has struck me that any imprisoned air 

 might be gradually liberated by the solvent action of the 

 benzole in the balsam, whilst the diatoms might still be re- 

 tained in their places by the stickiness of the caoutchouc under 

 these circumstances ; at least long enough for the balsam to 

 get properly hardened in the cool oven. I merely throw this 



