Dangerous acid fumes (notably nitrous fumes) are evolved in metal pickling, especially of brass articles (such as harness furniture, lamp fittings, church utensils, &c.), for the purpose of giving them a shiny or dull surface by immersion in baths of nitric, hydrochloric, or sulphuric acid. As severe and even fatal poisoning has occurred in these operations they should be conducted in isolated compartments or channels under exhaust ventilation. If the ventilation provided is mechanical an acid proof earthenware fan or an injector is necessary. The following description applies to one large works: The pickling troughs are placed in a wooden compartment closed in except for a small opening in front. To this compartment a stoneware pipe leading to a stoneware fan is connected. The nitrous fumes are drawn through the pipe and led into the lower part of an absorption tower filled with cone-shaped packing material through which water trickles from a vessel placed at the top. The greater part of the acid fumes are absorbed as they pass upwards and the water collects in a receiver below, from which it is blown by compressed air into the vessel above for utilisation again until it becomes so charged with acid that it can be used for pickling purposes. In _galvanising_ and _tinning_ acid fumes, injurious acroleic vapour, and metallic fumes can arise as the metal articles (iron, copper, &c.) first require to be cleaned in an acid bath and then dipped into molten fat or molten zinc or tin. Here also the fumes should be drawn away in the manner described. Recovery and Use of Mercury Escape of mercury vapour and development of sulphur dioxide seriously endanger workers engaged in smelting cinnabar. The danger can be minimised by proper construction of furnaces preventing escape as far as possible of fumes and most careful condensation of the mercury in impervious and sufficiently capacious chambers and flues. Continuous furnaces are to be preferred to those working intermittently. The system of condensing chambers and flues must offer as long a passage as possible to the fumes, and care must be taken to keep them thoroughly cool. Removal of the deposit rich in mercury from the flues is especially fraught with danger. This work should only be carried on after efficient watering by workers equipped with respirators, working suits, &c. _Use of mercury._—Mirror making by coating the glass with mercury used to be one of the most dangerous occupations. Now that a fully adequate substitute for mercury has been found in the nitrate of silver and ammonia process, use of mercury should be prohibited. As a home industry especially mirror coating with mercury should be suppressed. Fortunately the dangerous mode of production is rapidly being ousted. The following requirements are contained in a decree of the Prussian Government dated May 18, 1889: (1) Medical certificate on admission to employment in mirror making with use of mercury;