The background of this question is as follows. Sean Carroll is in the process of giving a series of talks recorded in his home office and published on YouTube. The setup is intentionally very informal. The overall name of the series is Biggest ideas in the Universe.
Recently, he threw out an on-the-side remark that really annoyed me. Sean Carroll was talking about Ludwig Boltzmann, and about opposition from some of his contemporaries.
Transcript 52:35 into the video:
[...] a lot of people in germany led by Ernst Mach of Mach's principle fame didn't want to believe in the existence of atoms. They thought that things like the second law of thermodynamics were not probabilistic. [...]
So, at the time, what was actually going on? What, in truth, was the attitude of Ernst Mach towards the existence of atoms?
The Stanford Encyclopedia of Philosophy has an article about atomism.
The striking thing about atomism in the 19th century: on one hand there is circumstantial evidence in favor of atomism, in the form of elements tending to combine in particualar ratios, and from kinetic theory of gases. On the other hand, for both lines of circumstantial evidence there were also anomalies. Cases were known where the chemical reaction doesn't show a preferred ratio, cases were known where certain experimental results could not be accounted for in terms of the existing kinetic theory of gases.
In the Stanford Encyclopedia article Ostwald is mentioned. Ostwald was of the opinion that the case for atomism was not conclusive.
Mach's philosophy of science
Mach's philosophy of science was very austere. At the time there was the school of thought called 'Logical Positivism'. As I understand it: in terms of logical positivism only experimental results and the formulas that express the structure of a theory of physics are regarded to fall in realm of science.
In this austere view even interpretation of a theory of physics isn't necessarily science, in the sense that this interpretation may be in part speculative.
So, I can see Ernst Mach arguing that the evidence available at the time does not prove atomism beyond doubt.
Specific question: of the people who opposed the views of Ludwig Boltzman, what was their opinion on the status of atomism?