Particle system stop?

I tried exactly that, but it doesn't let me drag the particle system into the inspector slot. As if it doesn't recognise it, even when I search for it using the node at the side.

I presumed you meant "ParticleSystem" with an "e" at the end. I'm not sure what else to try.

 public ParticleSystem particleSystem;

I'm not sure of your particular setup but that's how it's supposed to work. I've done this many times before. Are there any errors in your console? Also, you cannot select the script in your Project window and then drag the particle system onto the script's slot in the inspector. You need to assign that Bubbles script to a GameObject first. THEN you can drag it onto that instance of the script which is on the GameObject

Thanks for the advice again, I know you can't drag a particle onto an asset directly in the project window, I was trying it on the Main Camera, Particle system itself and an empty game object with the particle parented. For some reason when I've tried again with:

 using UnityEngine;
 using System.Collections;
 
 public class Bubbles : MonoBehaviour 
 {
     public ParticleSystem particleSystem;
     
     void Start () 
     {
         particleSystem.Stop();
     }
 }

I get a null reference again.

 NullReferenceException: Object reference not set to an instance of an object
 UnityEngine.ParticleSystem.Stop (Boolean withChildren) (at C:/BuildAgent/work/7535de4ca26c26ac/Runtime/ExportGenerated/Editor/ParticleSystemBindings.cs:496)
 UnityEngine.ParticleSystem.Stop () (at C:/BuildAgent/work/7535de4ca26c26ac/Runtime/ExportGenerated/Editor/ParticleSystemBindings.cs:488)
 Bubles.Start () (at Assets/Bubles.cs:10)

I also get this message when I save the code from MonoDevelop.

 Assets/Bubles.cs(6,31): warning CS0108: `Bubles.particleSystem' hides inherited member `UnityEngine.Component.particleSystem'. Use the new keyword if hiding was intended

So you have a MainCamera in your scene. That MainCamera has the Bubbles.cs script attached to it. Then you have a particle system in the scene, and you drag that particle system onto the bubbles.cs instance on the MainCamera? Is that about right? (Also I notice that your errors reference "Bubles" but the class you've copy/pasted is "Bubbles" with 2 "b"'s. Not sure if that's a typo...

That is correct, yes. "Bubles" was just my input error when I was replying. I was going to mention that but I forgot! :(