“To give a touch of concreteness to the discussion at this stage a very simple example may be given to illustrate how several equilibria may occur in a life-bearing system. A perfectly screened dwelling may be kept indefinitely free from flies. This is a condition of equilibrium, but of unstable equilibrium; for if only a few flies gain access, presently these will breed, and the room will become inhabited by a population of flies whose number will depend on the amount of food present, on the measures taken to combat the pest, etc. Unless these measures are very active, the flies will not be wholly exterminated, but the population will attain some approximately steady number (for a given season) . There are, then, in this case, two possible equilibria; one with a zero population of flies, the other with some positive number of fly population.”

Lang (1955) standing (Ѻ) in the doorway to the Carlsberg Laboratorium.

“The immense importance of applying chemico-physical methods to biology has gained world-wide recognition in recent years. Not even the most miserable scientific sparrow falls to the earth without the fateful god of mechanical statistics having something to do with it. Still, there are a few realms within which this power has not yet taken charge, and the present article is written with the purpose of showing where and how it can be applied in an entirely new fashion and whither this application may lead us in our struggle for bigger and better science. [1]

1. Problem: Measurement of activity of housefly.
2. Materials: Adult male flies. No particular strain. [N2]
3. Method: Osmotic. Suspension of flies in air (aerosol) enclosed in semipermeable cylinder with piston.
4. Results: Inconclusive.
5. Theoretical: Yes.

[N1] By special courtesy the present article was accepted without having passed the usual five referees. An American friend of the author has however pointed out to him that it is inadvisable to go the whole hog in the direction of broken literary English, and it will therefore be attempted—in places—to adopt the Standard American Scientific Language whose meaning will be clear even to the luniest [see: crackpot or madness].
[N2] Except from exercise.”

Lang's figure 1, captioned "The osmometer. Flies are left out for obvious reasons." Device, supposedly, is a play on the van't Hoff equilibrium box, a piston with a piston heads semi-permeable to certain types of gas molecules, designed and built in circa 1886 by Jacobus van’t Hoff.

“He was one of the most gifted, generous and lovable men I have ever met, and is remem­bered, indeed worshiped, by those who came in contact with him, particularly those who had the experience of working in his lab­oratory.”

— Fritz Lipmann (1980), commentary on Lang [4]

“One well established and generally accepted method of treating systems which are complicated beyond comprehension is to construct simple models and see whether they fit the systems in question. If they do, you will immediately become suspicious, and so will your colleagues most certainly, with the result that a blooming literature sprints up (or breaks out) dealing with the problem of how you have managed to make all your errors cancel one another. If they do not fit, the beauty of the models themselves may shine for years untainted by the squalid awkwardness of reality.”

— Kai Lang (1956), “The Thermodynamic Activity of the Male Housefly” (§:5.31); cited by Mogens Westergaard [2]

“Are flies comparable to macromolecules? Yes.”

— Kaj Lang (1956), “The Thermodynamic Activity of the Male Housefly” (§:5.312)