As early as 1756, Russian scientist lomonosov put forward the rudiment of the law of conservation of mass. He was the first chemist to measure the weight of chemical reaction with a balance. He overthrew the previous "phlogiston theory" and thought: "The weight of all substances involved in the reaction is equal to the weight of all reaction products. The reason why Holmes' smoking experiment must exclude "loss" is that some smoke must be inhaled by Holmes. In chemical experiments, some substances are transformed into other forms that are difficult to detect, and the resulting errors have troubled many chemists.