An American critic, disgusted with the mob in _Julius Cæsar_, when that play was acted lately at Booth’s Theatre, because they shewed no discrimination, cheering the meanest soldier walking in procession, while they let Cæsar and Antony go by unrecognised, insists upon the supernumeraries being better taught. It is certainly the duty of the stage-manager to see that they are properly instructed, but it is no use to ask too much of them; like the actor-manager who called upon his supers to assume an oily smile of truculent defiance; and the author of _Jeanne d’Arc_, who in his stage directions requires the representatives of the English spectators at the procession to the pyre to give vent to a buzz and murmur of hatred and exultation; and the representatives of the Amazon’s countrymen to express their feelings in a buzz and murmur of love, pity, and sympathy. Such exacting gentlemen remind one of the French manager who fined one of the supernumeraries engaged in _Paul et Virginie_ for not making himself black enough, and afterwards discovered that the man he had fined was a nigger born. THE MONTH: SCIENCE AND ARTS. The paragraphs on the use of zinc as a preventive of scale in steam-boilers, in the _Month_ for March last (_ante_ 207 [Transcriber’s note: see Project Gutenberg eBook 62970.]), have brought us many inquiries for further particulars. One correspondent wishes to know what length of time the lump of zinc will last? to which we answer, that on this point there is nothing more precise in the original Report than that the zinc lasts the usual time of working the boiler between the periods of cleaning. The zinc is more efficacious in the form of an ingot or solid lump, than when small heaps of clippings are employed; and we cannot imagine that it would be difficult for any intelligent person to determine by observation the dissolution of the zinc. The theoretical explanation of the preservative action is, that in the process of oxidation the zinc borrows oxygen from the air dissolved in the feed-water only. The two metals, zinc and iron, surrounded by water at a high temperature, form an electrical ‘pile’ with a single liquid which slowly decomposes the water. The oxygen flies to the most oxidisable metal, the zinc, while the hydrogen is set free on the surface of the iron. This release of hydrogen goes on over the whole extent of the iron in contact with the water, and the minute bubbles of this gas isolate at each instant the sides of the boiler from the incrusting substance. If the quantity of this substance is small, it becomes so penetrated by the bubbles that it remains soft as mud; and if in greater quantity, coherent incrustations are formed, but in such a state of isolation as to be readily separated from the iron.